Stays will probably be 525 or 725 for most builders. A lot will still have 531 fork blades too.

I would go for the Mercian personally, of the two. Their turn around time is faster. You could also look at folks like Pennine, Woodrup, Ellis Brigg around Leeds, or JF Wilson in Sheffield. Witcomb do some nice frames in London.

I have been thinking of similar things, except with a cyclocross slant. I'm also thinking about the Simoncini frames that Cycles Clements imports. You could also visit Condor. Most of their frames are made in Italy, but my friend raves about her Fratello