I did some mulling on this frame today (not that I've nothing better to do, mind you) and suddenly remembered I have these two in my attic:
Their bottom bracket shells look like this:
Look familiar?
These frames were made by Miki, of Sakai, Japan. I did some checking and the seat cluster, fork curve and rear drop-outs of your frame closely resemble those of a Union Sakai, a low-end offering from the Dutch Union brand, with frames that were made by Miki.
Miki is known to have supplied frames to many bike manufacturers all over the world, quite possibly also to one or more British manufacturers. The light mount braze-on does lead me to think it was made to British specifications.
If so, I think it was made in 1980, between the 24th of March 24 and the 5th of April. A good quality low-end frame.
So, combined with the fact that there are no decals on your frame, and it looks to have been repainted at some point in time, its current market value is limited.
If I came across this frame set at a swap meet for say €30 I'd probably be tempted, as it looks to be close to my (62cm) size, and I'd be curious as to its history, and see if I could make it into a nice British style three-speed club tourer. However, as I'm in Holland, shipping costs will probably prove to be prohibitive.