I think @juvela is (as usual) spot-on here. For evidence, I have these photos of what I believe to be a very late '70s/early '80s Tim Neenan Lighthouse. The fork IMMEDIATELY rang that bell for me -



Whilst the BB shell is not identical, it just puts out the same sort of vibe, you know?




Forgive me the multiple photos of the right rear dropout, but I wanted to try to capture an angle close enough to the OP's to show the similarity. This, too, jumped out at me when I saw the post.





Here is the serial number pic of mine, no. 1631 - though how he did his serial numbers is a mystery to me, as I have never been able to find a way to get a response from him. Maybe all my emails, etc., are out of date.



Finally, some pix of the bike as built - in a much more pedestrian 60cm or so size, which is (hallelujah!) my size.





Mine still bears the remnants of its Reynolds 531 tubing sticker. My understanding is that Mr. Neenan switched over to Columbus tubing relatively early on. Mine has lugless fillet-brazed construction except for the BB shell, and later Lighthouses are fully fillet-brazed or fully lugged, but not hybridized like this one. Mine came to me drilled for traditional nutted brake calipers, but I opted to drill for Allen key, so there's that. Rear spacing on mine is 126mm, but they were always custom, one-off bikes anyway, so that proves nothing. I DO sometimes wonder if mine was built for 27-in wheels, as there is clearance for 32mm tires AND mudguards. And mine has the under BB cable routing combined with the ancient Campagnolo diver's bell cable housing stop on the underside of the chainstay - which requires a funky ferrule, and I could see Neenan concluding that's more trouble than it's worth. I think that's classical red Imron paint on both mine and the OP's bike, so likely original - and in my search for info on Lighthouse bikes I've seen several examples with this precise color and general style.

If the enormous bike fits, AND it is indeed a Lighthouse, it is a joyous experience waiting for someone. I adore mine, which has turned out to be pretty much what my Rivendell was supposed to be and never quite pulled off - but that's another rant for another day.