Tried it with my Pace. You get into a few problems. If you put your historical glasses on then most of the world can loosely be called 'British'. This includes France - Mavic; the US so all manner of things become available. Sadly Italy was never part of the Empire so Flite saddles ca't be included. Neither was Japan or Hungary so that means no Shimano.
Here's my list frame, forks, stem - Pace
Headset, hubs - Hope
seatpost, bars & bar ends - USE
cranks, chainrings - Middleburn
(saddles can be done - USE, XLite or Brooks(?))
Loads of British brands are actually imports e.g.on-one, the more recent Orange hardtails etc... if its a British brand, but made abroad is that British? Are Marins British or US?
Ruminate fiercely