Ficus carica, the fruiting fig, is hardy in Britain, but should be planted against a warm wall (south facing preferably). May remain evergreen in mild areas of the country in mild winters, but loses its leaves in cold winters and colder areas.

I would be amazed if a fig stayed evergreen in the UK, Bamboo. They lose their leaves every year here, and we often have years without any frost at all. The only evergreen fig that I would rely on in the milder areas of the UK is a vine, not a tree--Creeping Fig (Ficus pumila). There may still be a species from high altitudes that I don't know about, since it won't grow in the hot desert.

I had a relative in South Devon whose fig surprisingly retained some leaves during winter - but then they had a micro climate there, and it was in an extremely sheltered, walled garden.

Maybe ours drop the leaves more from the difference in day and night temps, too. In January, it can be 27º C in the day, and 1.7º at night!