It is important to realise that, like many agricultural crops grown in a variety of climates, be it wheat, apples or tea, C sinensis comes in many varieties, with different climatic tolerances. On the one hand, C sinensis is grown in parts of China and Japan with a winter just as cold as many parts of the UK. On the other hand, it is grown in parts of Assam, Sri Lanka, and Kenya, where the local winter is about as cold as a Cornish summer, but drier. It is even grown in equatorial climates -albeit only at altitude - such as the highlands of Malaysia and Rwanda, where they don't have a cool season at all.

So I think the key to success with growing C sinensis in the UK is finding a suitable variety, and giving it a lot of water in the growing season.

Coffee also has its varieties, and although the growing regions of tea and coffee overlap (eg highland Kenya), it is not capable of growing in places with anywhere near such cold winters as many places where tea is grown.

The native region for tea are the hills along of SW China (ie Yunnan, not Tibet) into NE India, which, because of altitude, is on the edge between warm temperate and sub-tropical climates - not entirely frost-free I would suggest. Whereas coffee comes from locations at similar altitude in Ethiopia, but being a good 10-15 degrees further south is more likely to be frost-free.