There aren't that many manufacturers that are specifically UK-based, but there are a few retailers with a good selection. The online Apple Store stocks a few brands.
Laptop Stuff has a few different laptop bag sections. And
eBags has a UK presence and carries all kinds of bags.
If you're into leather bags,
Knomo is one manufacturer that's based in the UK.
I'd recommend trying out bags in person, if possible, particularly for the 17".