Re:
If you have serial no.63410 it might be reasonable to provisionally date the frame to 1963, unless there are any glaring anachronisms? Is the number on the steerer too?
Off the top of my head- builders who (iirc) used the 'first two digits=year of build' number format: Mercian, Pennine, Pat Hanlon, Jack Hearne.. by no means a complete list.. Probably not many on the 'complete list' put the number on the dropout, so that'd narrow it down.
That fork crown was commonly used afaik at least between mid '60s to mid '70s. My go-to internet exemplar was the '64 Jack Hearne at Classic Lightweights, and I have a putative '75 mystery frame with the same casting. There's a Major Nichols website with examples of frames with that crown, and it is referred to there as being a Milremo crown. I'm not sure if 'Milremo' was simply a Ron Kitching rebranding exercise, or if there really was a Milremo factory somewhere- probably not in Milan or San Remo or anywhere in between- turning out hubs, rims, stems, saddles, and fork crowns?
The driveside rear dropout on your frame seems to have significantly more 'web' behind the derailleur-mounting hole than I'm used to seeing on a 1010- more like a Simplex dropout. I'm not saying they are not 1010s, just that I've never before seen one quite like that.
The lugs look more Prugnat than Bocama to me..