237SJ:
Please don't confuse 'British wine' with 'English wine'!!!
The law permits producers to sell wine as 'British' as long as it's bottled in the UK. As I've stated above, such products are usually revolting. (They're also inconsistent. You might buy one bottle which consists mainly of French Cabernet Sauvignon wine, whereas the next bottle of the same brand can be mainly Greek wine from Xinomavro grapes).
However the law only allows producers to call wines 'English' if the grapes have been grown here. They're almost universally of good quality but, because of low crop yields, often significantly more expensive than comparable wines from sunnier countries.
Chris