I saw a lot of Jacksons and Mercians etc growing up. They are nice, and the low pound is really helping you out atm. So, ordinarily, I would have suggested the Jackson. But... I've since spent a lot of time with an old Marinoni road bike. The bike is fantastic (I prefer it to my Colnago) so I would go for the Marinoni
There are plenty of other UK builders though. If you like disco paint, you should get a JF Wilson (that was the one I wanted!). Steve Goff and Woodrup would be two of the cheaper builders (about the same as Jackson).
Anyway, either of those two would be ace....