Gillott track. The sentimentalist.

Nice frame here!
There should be a 'T' shape cut near the top of the steerer column if it was built for a head clip. The clip will pinch the steerer column as you adjust it and will hold the stem shaft in place.
Not an expert but the fork crown race does look a bit too chunky.
Headset top nut looks to be alloy, and if it is, most probably be Lytaloy Hobbs of Barbican.

I am going to do some more research. It may use a headclip headset, if so it would place the frame 30's or 40's.... saying that, it seems to fit together in the correct way, though the bottom race seem to protrude quite far.....It has cups that sit in the lip of the head-tube that the ball bearings go in....... not sure i'm describing it right.......never seen one like this before.