It's amazing just how difficult it is to source fine used GB these days. Even common definitives are hard to find, but the commemoratives are even harder.
There are several reasons for this. The first is simply that many POs don't even stock commemoratives, and many others claim to be too busy to sell them. I once needed some in a hurry and used a different PO than usual. They explained that they did have some commemoratives, but that it would take their safe at least a half hour before it would open! It is ridiculous that stamps designed to publicise and promote the UK are hidden away in dark corners and most returned to be destroyed. In the sixties there would be queues outside post offices every time a new commemorative set came out BECAUSE PEOPLE WANTED TO USE THEM!! Whenever I leave parcels at my village PO other customers love the stamps I use for the postage and wonder where they can get them from! There is not even a poster publicising new issues! The modern PO does not have a clue about marketing - they would rather sell everything via the Philatelic Bureau because the accounting is easier!! Yet they have thousands of outlets throughout the country with many customers who would lap up the stamps if only the knew about them and were easily available. This is almost free money for the PO, and would be a great boost to stamp collecting.
The second reason is that even where commemoratives are available in many cases they receive a stupid killer postmark or, worse, no postmark at all. And how many POs still have nice hand cancellers and how many PO staff have the pride in their job to actually apply neat postmarks? I am very lucky that my local PO at Pensford love my business and always try to postmark my boxes and letters well. It may well be that the Pensford postmark will be one of the few genuine ones that collectors will find on the small amount of fine used material on the market.
The third reason of course is that whatever does go through the post needs to survive intact! I protect my packages with plastic over the stamps but a lot of dealers these days don't bother, some even use couriers or scrap recycled stamps.
Some of the very rarest issues of all will be the prestige booklet panes (almost miniature sheets in themselves). Prestige booklets sell at a premium but are listed by Gibbons as the earlier ones didn't. They are of course mainly collected in mint condition. Ask yourself just how many actually see real use! I do use them occasionally on the larger kiloware lots and sometimes I'm lucky enough to get to buy them back! Needless to say when I do get them they sell very quickly - the one above with the 3 Festival of Stamps 1st and 3 £1 values sold within a minute or so when I listed it this morning! But how many are out there, with genuine postmarks used to send a real package? Ten thousand? A thousand? A hundred? Ten??