The 3 or 4 Hours is important in relation to EU regulation for delayed flights of over 3,500Km (and 2 or 3 hours for 1,400km).

In the OP's case PRG is about 1000Km from LHR, so the 2 hours benefits kick in.

http://eur-lex.europa.eu/LexUriServ/...01:0007:EN:PDF (page 4)

Of course, the OP may prefer to look at this: ECJ: Delays Could Cost Airlines

However, note that this is undergoing review and you may be waiting a long time.