The Lightning did come in that "green sunburst" colour scheme, but I don't think that's one. My brother had a Lightning in about 1980 (in "burnt cherry") and the frame wasn't 531. It's true that 531 stickers can be bought, but that one looks original - or at least, like it's been there a long time.
My guess would be a Dawes Jaguar from the early eighties, but I can't be certain. The Jaguar had 531 main tubes, but cheaper forks. Other bikes in the range either had 531 forks or cheaper materials throughout. Is the 531 sticker legible?
There's a 1982 catalogue here:
1982 Dawes Catalogue 1982 Dawes Catalogue (source VCC Library) I was not sure about the year, but after some consideration it had to be 1982. There are a couple of pointers: the black oval head bad…
dawescycles.wordpress.com
It's not a perfect match for your bike, but models would change from year to year. You may find a date code stamped on the rear of the brake calipers if they're Weinmanns.