Ignore the numbers for now. The diameter of the seat post will tell you more about the frame with a lot less effort. 27mm or 27.2mm is Reynolds 531. Around 26mm is Reynolds 501 and 25.4mm is normal grade steel. 18-23 is a marketing thing from Raleigh which refers to their normal grade steel. Most Raleigh's share almost identical geometry, hence they all ride the same and it happens to be quite a stable pleasing geo whether you have a gas pipe bike or a top end model. They really got it right.
Show us a full picture of the frame and it'll tell us more about the era, then you'll be able to narrow it down from there. Easy to do that from what is brazed on, which lugs, fork shape and rear spacing. We can already see that it has cast dropouts, not pressed ones, so it's not an entry level model.
The numbers will tell you the exact model, year, and where it was made, but probably better to ask on Retro Bike than on here, because there are a couple of folk on there who know the numbering scheme.