There are many low cost airlines operating on the way from London to Prague, so you can regulary find cheap flight ticket from London to Prague.

Curently the cheapest deal is with Wizzair for 29EUR from London Luton Airport to Vaclav Havel Airport in Prague.

Tue 8 Sep 20:55 London (LTN) with Wizzair
Tue 8 Sep 23:50 Prague (PRG)

Mon 21 Sep 21:15 London (LTN) with Wizzair
Tue 22 Sep 0:10 Prague (PRG)

Also cheap deal is with Ryanair. For 36 EUR you can get to Prague from Stansted Airport

Tue 8 Sep 19:35 London (STN) with Ryanair
Tue 8 Sep 22:30 Prague (PRG)

Tue 29 Sep 6:45 London (STN) Ryanair
Tue 29 Sep 9:40 Prague (PRG)

For less than 60EUR, you can get flight ticket also from Manchester Airport with stop in Brussels

Tue 29 Sep 7:50 Manchester (MAN) with Ryanair
Tue 29 Sep 10:15 Brussels (CRL)

Tue 29 Sep 13:10 Brussels (CRL) with Ryanair
Tue 29 Sep 14:40 Prague (PRG)