I think it might be French, or at least constructed from French parts/tubeset. The seat tube being undersize (vs a standard tubeset) is something of a giveaway. If the tubeset is a lightweight one the seat pin is usually 26.6mm in a French tubeset.
If it was built in the UK (or for the UK market) then the BB threads and headset threads could be BSC, still with French dimensions elsewhere.
If there is a frame number this might give a clue to the frame's origins. It might be worth posting details on a French cycling forum.
If the frameset (frame and fork) together weigh around 6lbs then the frame is reasonably lightweight (comparable with 531DB more or less). However I suspect that it may be a little more than that; the frame appears not to have been hand-finished to any great extent. For example the rear dropouts have not been blended where they meet the chainstays and the lugs have not been filed except on the points; these are not usually the hallmarks of a first-class frame.
BTW it may just be an optical illusion (aided perhaps by the fact that the seat angle appears to be nowhere near parallel to the head angle) but your photo of the head tube makes it look as if the frame might be twisted slightly. Worth checking that.
The brake bosses are almost certainly Mafac ones and back in the day it was normal to use the pressed steel Mafac rear hanger (rather than a braze-on of some kind) because it was lighter (and cheaper) than anything else. [Although a few racers would actually drill the seat post and use
that as the hanger...
] The gear lever bosses look a lot like Simplex ones; if they are
old simplex ones they may originally have been tapped M5 x 1 rather than M5 x 0.8.
Old CX frames can make nice training/do anything frames; however they are often built so light (and with steering geometry so quick) that they can't be loaded up without starting to steer poorly. Also the wheelbase is usually kept short, so that once you fit mudguards (using clever adaptations if you don't want to drill anything), there is nearly always toe overlap, even on quite large frame sizes. The other feature that can be a blessing or a curse is that the top tube is often kept shorter than normal for any given frame size.
BTW if you want to fit mudguards or a rack to the rear, P clips may suffice but there are also options involving adaptors that fit into the triangular cutout in the rear dropout. Blackburn used to make such things.
cheers
