When my father passed away, he left an extensive and very specialised collection of modern British stamps. (e.g. he might have had dozens of what initially looked to be identical, very common stamps but with every possible variation in colour, perforations, watermarks, phosphor bands, etc). I knew that the SG catalogue value of just a single one of his many albums of stamps was many thousands of pounds (although I also knew that stamps often change hands at only around 10% of their SG catalogue values).
So I carted the whole lot down to Stanley Gibbons in London, to see whether they were interested in buying them directly or (more likely, I thought) accepting them for entry into one of their auctions. They were neither interested in buying them nor offering them at auction. So I ended up taking them to a rival dealer, just across the road, and accepting just fifty quid for the lot (despite knowing that the SG catalogue value was in excess of £10K).
That was a quarter of a century ago but, unless you've got some extremely rare stamps, I doubt that you'll get anywhere with Stanley Gibbons either.
Of course that was before internet auctions became popular and that's where I'd be looking if I was in a similar position today. The world's leading auction site for 'collectables' (including postage stamps) is Delcampe, which currently has around 25 million philatelic items listed. (I use it for buying postcards and it's one of my favoure websites).
I strongly recommend taking a look at Delcampe. Even if you decide not to sell there, it will give you an indication of the prices that your stamps might actually fetch at auction (rather than their rather meaningless SG values):
https://www.delcampe.net/en_GB/collectables/