Gotta agree with most people on here and say that there's no such thing as a dead cert.
For you to get a game that pays £37 500 from a £25 000 bet, you'll be looking at odds of 1-2 .... the bookies will have it at them odds for a reason .... plus the odds will be slanted in their favour to allow for profit, etc.
In other words, the bookies will consider the odds of the team winning to be more .... if you placed a number of different bets at 1-2 - overall you'd end up losing.
The best analogy I can give is the flipping of a coin and betting on if it ends on heads or tails. It's a straight 50/50 bet, but if you could get odds for this at the bookies, you'd get slightly less .... I'm just guessing that they'd offer you 2-5 or 9-20 (just guessing, so don't quote me).
If you bet on this as a one-off, you may get lucky; but the more you keep betting, the greater the chance that you'll lose, as the odds are against you.
My advice - stay clear of "dead certs".