I've checked the route information on the Prague Airport website (and used it as the basis for researching an answer to your question). Unless there's an error on that site, the information below should cover every route from England to Prague:
Jet2 fly from Leeds/Bradford. They're currently offering �49 each way + taxes & charges. That comes out at �123.48 per person:
http://www.jet2.com/
easyJet fly from Nottingham (East Midlands). Currently �57.59 out, �37.49 return. With taxes & charges (and rejecting the insurance which their website tries to add on) the total is �118.58.
The easyJet flights from Gatwick, Stansted, Bristol & Newcastle are all well over your budget.
BmiBaby's flights from Nottingham and Manchester are also too expensive for you.
ThomsonFly can take you from Doncaster/Sheffield for �34.49 out and �44.49 return. With the extras, the fare is �113.78 per person.
BA's fares from both Heathrow & Gatwick are too expensive.
CSA's fares from Heathrow, Stansted & Manchester are all well over your budget.
So, it seems that you can't do it for �100 per person. (That's not surprising for an 'out Friday - back Monday' trip at fairly short notice). However, if you can make your way to either Yorkshire or Nottinghamshire, you can still get reasonably priced flights.
NB: I did similar research for another question a few days ago. The prices for that journey were correct when I posted them in the early hours of the morning (like this post) but they'd gone up by �32 at tea time on the same day. I strongly recommend booking as soon as possible!
Chris