I posted earlier about many of my postcards from Scotland and England not arriving to the recipients.\ in a timely manner. It turns out that most have now been received recently -- 3 months later! They have postmarks showing they were sent from the Philippines! The Royal Mail hasn't answered how this happens, but I suspect that a handwritten note declaring "Air Mail" is not sufficient or perhaps the International stamps that can be purchased in souvenir shops are not actually sent by air. In any case, I highly recommend buying stamps in official post offices and also getting the blue "Air Mail" stickers that they provide. Good Luck!
This also happened to me, and I posted the question on tripadvisor and got a surprising explanation. I had one stamp left, so I was able to verify I had purchased it in one of the Historic Scotland shops, and it says it was printed by Universal Mail. I have written a complaint to Historic Scotland. I think it's a very sleezy way to make money from tourists! Here is what was posted. Peggy NC
Are you absolutely sure that the stamps were purchased from a proper Royal Mail Post Office and not just a shop that happens also to sell stamps? Since 2006, the Royal Mail has no longer been allowed to have a complete monopoly on deliveries and that has led to independent companies setting up alternative services that do not use the same delivery routes as the Post Office proper. They produce colourful or gaudy stamps with resemblance to the 'real thing' which are generally found for sale in tourist areas, and particularly in shops which also sell postcards.
The stamps can only be used for delivery of postcards to addresses outside the UK but these kinds of companies use sea transport to lower the cost, cheap foreign labour for sorting, and your postcard could literally go anywhere in the world that has a coastline before it eventually reaches you. The main routes are through SE Asia. I have no explanation for the felt tip pen alterations... unless it was done to ensure that some kind of scanner with word recognition software didn't pick up the word 'Scotland' and result in it being sent back there.
There are several of these companies, of which "Universal Mail" is probably the biggest. They have no connection at all with the Royal Mail Post Office and the sellers of these stamps are supposed to explain that to you (but of course they often don't). If your stamps looked anything like these examples, then you've been had: By agreement with the Royal Mail (reluctantly), postcards with these stamps can be put in any standard post box... but they are machine sorted to isolate them and they don't then go though the Royal Mail network. They are sent to a site operated by the company that produced the stamps, for them to independently arrange delivery to destination.
Real British stamps have Betty Windsor's head on them (click for photo). Accept no substitutes.
I think a British person would have complained if they were given non-Royal Mail stamps. Sounds a bit like the DCC problem.
(P.S.for those foreigner's who don't know, Betty Windsor lives with her husband Phil the Greek in Buck House)