I've been thinking about this for a while, that we should have a sticky thread of British manufacturers. Just an idea. In any case I'll get the ball rolling with some I know of.
http://www.crojack.co.uk/
Very nice selection of outerwear and personal items, I've got a cord jacket from them
www.sehkelly.com
Makes fancy, rather eclectic garments from all-British materials
http://www.old-town.co.uk/
Vintage clothing, some cut from actual 19th century patterns, fits in surprisingly well with the 21st century it seems
http://www.privatewhitevc.com/
General menswear, rather pricey
www.oswaldandkane.com
Makes casual shirts
http://www.originalmontgomery.com/
Makes original pattern duffel coats
http://www.coeurdelion.co.uk/
Basic range of menswear
http://www.lavenhamjackets.com/
Manufactures nice quilted and country jackets

Sundries:
www.hj.co.uk
Socks
www.kiniki.com/‎
Underwear and swimwear
www.dapsquared.co.uk
Boxer shorts
Some directories:
www.madeintheseisles.co.uk/‎
makeitbritish.co.uk/‎
www.stillmadeinbritain.co.uk/‎

I know more manufacturers but not off the top of my head, feel free to add to the list!