Ourarka - both the clematis and any grape vine would be happier in the ground rather than in pots. Do you want fruit or are you content with leaves and Autumn colour? (eg 'Brandt') Either way a grape vine will grow pretty big and will cover your pergola densely unless you prune it annually. They are also prone to mildew unless sprayed. It will outgrow and smother the Clematis unless yours is C montana or one of the other vigorous types. Best bet is to research grapes on UK sites for suitable cultivars and pruning advice. I see you have an olive already.

Here is a link that might be useful: Outdoor grapes

Yes, the olive went in in the Spring. I wasn't intending on the grape sharing the space with the clematis as I was going to get rid of that. However, if you don't think a grape would work .......

With the exception of just a couple cultivars, neither clematis nor grape will do well in containers.......unless much larger containers than you now have. FWIW, most vines that produce significant topgrowth (like enough to cover your pergola) will produce equally large root systems that make growing in a smallish container problematic at best.

It looks like you have room to plant in ground just to the left side of the pergola. I'd consider an ornamental (rather than fruiting) grape for that position - something like Vitis vinifera 'Purpurea' or Vitis coignetiae, both of which produce stunning fall color. You could get grapes forming on either but not particularly tasty ones. As flora has indicated, fruiting grapes need to be selected for site appropriateness and a more local source is better able to provide that. Nothing wrong with growing fruiting ones but they do tend to require more attention than the purely ornamental species.

If you DO have the opportunity to plant in ground, nothing is stopping you from planting just about any sun-loving vine you want......clematis, grape, honeysuckle, even climbing rose.