It's a stamp which says 1st, 2nd, or E, and is always valid at the current first, second, or Euro-class rate, regardless of what price was paid for it.
- 1st: 27p (all UK, up to 60g)
- 2nd: 19p (all UK, up to 60g)
- E: 37p (all Europe, inc. Ireland, up to 20g)
nine9 - care to take Royal Mail to court, then? :) It used to be the case that a 1st Class stamp (at the time a mere 25p) would take a letter all over the EU, but no longer. I wasn't aware that such a law existed, but you'd think that a postal service sould know...