goto a large stamp fair and go round the dealers and see who offrs thebest price i recently got £500 for four old albums i had from schooldays. still have a further two albums left may flog these one day but they are full of nice colourful sets from kgv1 era
While effectively being worthless bits of paper their value is based on printers errors and rarity.
When my Father sold his Collection of Chinese Stamps. 7/8th of the collection went for pennies but a block of, I think, 11 stamps which had a printers error, bought us a good sized swimming pool, a Tennis Court and a top of the range Humber car.
That is part of the problem. Unless you really know what to look for, you are likely to have no idea of the value of any particular stamp. The Stamp Collectors who spend real money are looking for esoteric differences that few of us would know about, let alone notice.
Oh the reason that particular block of Stamps was valuable. There was a printers error that even after it was pointed out to me I couldn't see.
A specialist Auction Room is the place for special stamps. I don't think that Stanley Gibbons are the only player in the market. I am sure that a visit to them would be useful.