That'd the lower half

That's the upper half.

The soil is heavily compacted and there is a layer of salts
What type of pot plastic or terracotta?
My baby jade I got several months ago is in a terracotta with gravel top dress. Waters when leaves start to shrivel.

Specimen is very gaunt, these often need more light than typical interior rooms have to produce the normal appearance of an outdoor example in a suitable climate. I would stick it in the brightest possible exposure, supply artificial light if necessary. When suited these actually produce a generous display of fragrant white star-flowers during the winter.

Rooting from cuttings should be quite easy, I have seen these actually generating aerial rootlets from the stems - when this is happening you can cut future propagules that are in a pre-rooted condition.

As with other succulent plants probably should leave cuttings sitting out in the air for a few days while the cut portions react to being exposed, then insert in rooting medium.

Same basic types of potting mediums and fertilizers for this as other cacti and succulents. Specialist collectors will have their preferences for specific different plants but for your jade you should do find just buying pre-made products that are labeled for cacti and succulents in general. You might like to do a general web search for "growing jade plant", "jade plant cultivation" and so on.

I believe this plant is Portulacaria afra. It likes lot of light, shallow pots with good draining soil and temperatures 15-25 ðC. Fresh air and regular watering during growth season.
For any cutting/splitting into 2 parts wait for spring, now is not good season for it. But definitely cutting encourages branching and the plant grows more leaves after it.
Soil - I use mixture of bag soil, fine grit and quartz sand. In plastic pots and my plants grow happily.

I'd agree with Raxidor. That looks a lot more like Portulacaria...aka elephant bush/tree/plant. A pretty impressive one, at that, despite the lack of leaves.

Full sun with very fast soil and light but consistent fertilizing should do fine. If it's anything like mine are, the roots are rather impressive for a succulent. I take mine out regularly to root prune and stick back in, but without it, it'd need a bigger pot.

If it were mine, I think I'd leave it that way and just repot it. It'd be really striking & interesting with tiny leaves forming all over once it comes back into season. Otherwise, in your case, I'd lop off the tops and any branches not growing vertical. Leave them out for a day or two then stick in the same pot or a separate pot.

Can't help with bonsai but you can ask Al (aka screen name tapla; aka bonsai master; aka our very own gritty mix professor). I don't know that I have ever seen a portulacaria bonsai.

Must be getting rather nippy in the UK now and your rains are no joke, haha, so keep it warm and in the brightest light you have. I find my portulacaria take/accept more water than my other succulents (all in the same loose mix) without suffering at all. But I'd hold most water and fertilizer until early spring if it's indoors.

For soil, anything that drains fast should work well. (Bark, gravel, perlite....) There's a ton of info if you search for "gritty mix" or "Al's Gritty Mix." I've never had a succulent go wrong because of gritty mix....and 100% of the time, it'll do much better than regular potting soil.

Nice looking plant. Great find and gift! I suppose there is a slim chance that a jade in the garage (which I've never done) will change its stems and leaves to look like that...so I won't say I am 100% sure it's not a jade. I'm about 98% certain. :-)

Grace

Oh wow. So much fantastic information thank you both I really thought it was a jade.

So shall I remove some of the current soil and refresh it?

I have a grow light so I can provide like
It is raining every other day! And it's now between 4c and 12c leaves are dropping and finally going red. Still no frost.

I need to buy some new cutters (can't spell the french word haha?) there are a few branches that are weeping (lost its upright strength,) will they regain it with care?

During winter season I let mine plants in a bright dry place with temperature around 10 ðC (garage window for example). And I water them a bit sometimes when sun shines to prevent excessive leaf dropping.
I wouldn't stress the plant before wintering, so no repotting or cutting. Keep the plant as strong as possible till spring. When it starts to grow, repot it. After few weeks, when you see new growth, you can start cutting it. Optimal is to let 2-3 segments (pairs of leaves or just bare segments) from terminal branches. I noticed the plants take it better, when you don't cut all branches at once. Cut 1/3 of them, wait for new leaves and then cut another 1/3. Branches growing from wrong places or growing down can be removed completely of course. Try to google "portulacaria bonsai" to get better idea how to shape them :)

Adding a photo of my own "bonsai in progress"

Crassula are Winter growers. I live in zone 10 I trim off the tips of the branches leaving very few leaves on the tip of each growth I do this on the 15th of Oct. every year, then it will flower for Christmas, now is a good time to repot, but not start new growth from the cuttings. Fertilize it now after you change to a larger pot and if you have roots water do not pack down the soil, it needs the air in the soil. Remember that I grow my plants in So. Calif. Use no peat moss in the soil in Africa they live by the thousands in rocks, on top of rocks in the cracks, on the cracks

East side of the continent. they are telling us that that they don't need your help and know how to take care of them selves. I am pretty sure that it is an Crassula agentea, name but it has the smallest leaves of the "Jades".

This is not a Crassula, nor is it in the Crassula family. It is in the family Portulacaceae.

As others said, this is Portulacaria afra. Raxidor, these are sold around the world by the name "Jade" or "Baby Jade" or "Mini Jade," and not your fault in the slightest for the confusion.

Lots of light, water, and fertilizer during the growing season....but don't neglect to water during the Winter, either....or they'll shed leaves immediately.

Your bonsai in progress is excellent.

Josh

Oh fantastic bonsai,

So are you all agreed it not a jade?

At moment it's in a partially direct artificial light. No room on window cil
I'll be removing a small amount of the soil abd replacing it with fresh but a very small amount!! Can I cut a few inches of the root ball and put new soil in its place?

I would be watered half but at the same times.

All this information has been so helpful thank you.
Oh what strength feed?
I have flower encouraging cacti feed (1.1.3/1.3.1 which ever. It's a very low one)
I also have baby bio citrus and general plant feed 7.4.5
African violet feed 15.30.15
Grow more 27.35.19
Mirical grow same as above with trace
And tomato feed (unsure on the ratios)

Of the fertilizers listed, I would go with the 7-4-5....but reduce the strength to 1/4 dose.

Josh