The leaves look quite glossy? Is there any colour on the stems? How much sun does it get?
My guess, based on not a lot really, would be some kind of pepper or chilli.
In a sheltered spot many can survive a mild UK Winter

The stems are a sort of reddish brown. The plant gets about 2-4hrs of morning sun. The leaves have quite a firm texture

I don't want to lead you down a blind alley, I may well be completely wrong. The way the leaves are coming off the main stem doesn't look quite right.
On the other hand the red brown colour on the stems & the slow growth fits peppers. If no-one else corrects me, my suggestion would be to dig it out with an inch or two of soil beyond the leaf spread. Then put it into a 10-12 inch pot with gritty, free-draining compost & put it in the sunniest spot you can find!

well I'd go for something like osteospermum, its not like any chilli that I grow.

but perhaps someone else will have another thought on it.

I really wasn't confident Seaburn, hoped someone would have a better idea.

It looks like an Osteospermum to me, too, though they are very fast growing, and rarely survive the kind of winters that the UK gets.

I think it is an osteospermum, I have some more mature plants in my garden. They might have self seeded. It also might grow slowly because it's not getting enough light.

They are sun lovers!