England hosted the football world cup in 1966, and, as the English like to remind us Scots, happened to win it. A set of three stamps had been issued in advance of the championships. After England emerged as victors on 30 July, the Post Office decided to celebrate the win with the issue of a special stamp, and simply re-issued the existing fourpenny stamp with the inscription "ENGLAND WINNERS" added to the plain white area at the top. A common misconception is that this was an overprint on existing stamps; in fact this was an entirely new printing, the new black printing plate having the new text as well as the other bits of black print.
 

The original stamp. 146,162,040 were sold (ordinary and phosphor combined).  If an optimistic 5% survived, this leaves about 7.3 million on the market.

The "Winners" stamp. 12,452,640 were sold (issued non-phosphor only). Virtually every one went into the hands of collectors and dealers. At 95% survival, one could expect there to be almost 12 million remaining.

The stamp went on sale on 18 August. The Post Office had made it known that this new stamp would be issued in smaller numbers than normal commemorative stamps, and this caused excitement and speculation. Misplaced, as it turned out. When London's 24-hour PO at Trafalgar Square put them on sale, they sold out quickly, and word spread that these were "good". Queues at virtually every post office in England (as well as Guernsey, Jersey, and the Isle of Man - but not in Scotland, Wales, or Northern Ireland) snapped up supplies very soon after they went on sale. Many collectors were disappointed to have missed the chance to get one for their collections. Dealers with orders to fill were unable to get their normal supplies. Enhanced prices started to be asked - and paid. Within days, people were paying 15/- per stamp - almost fifty times the original cost. For a short time, sheets were even quoted on the London Stock Exchange !  Many people, seeing that their investment had been wise, held on longer. Of course, many who would normally have kept a solitary example for their collection had taken the opportunity to buy blocks or sheets.
After some time, people started to realise that the stamp was not scarce at all, and that there was more than enough to go around. Prices came down.
As a footnote, Scottish collectors were miffed at the fact that they were unable even to have the chance to buy the stamps. They might not have supported the team, but they still wanted to buy the stamps !  The post office somehow managed to find some further supplies, and these were put on sale at the Edinburgh GPO on 22 August.

Better items to look for. Basic stamps are common, even sheets often appear. The listed varieties have a slight premium (by the way, slight colour shifts, giving a white "halo" to the boot or other parts of the design, are fairly commonly found) but not high. First day covers are moderately common, but FDCs with some particular postmarks are more valuable - look for the Harrow & Wembley ones, or double-dated FDCs (set with 1 June and Winners with 18 August, on the same envelope), or the unconnected special handstamps in use that day for the Methodist Conference, the Opening of the Tay Road Bridge, or the Ballater Games.