Contract escape period

If something is not up to scratch - i.e. not satisfactory within the contract, then you could claim that the contract has not been fulfilled by the other party and cancel it without penalty.

There are more specific regulations too. The distance-selling regulations allow you to return any item purchased at a "distance" - i.e. without going into a shop and buying it - within ten days (I think), for any reason and without penalties. I know this applies to physical goods, but I don't know if it also applies to contracts (such as mobile phone plans).
Afaik, if you arrange for something to be delivered from a shop you should also have the right to cancel the delivery for a full refund since the contract has not been fulfilled until you accept the goods.

There is also legislation dealing with loan agreements. Again, I think the term is ten days - you have ten days from the date of signing the loan agreement, where you can back out without penalty. The bank will not provide you with any money until those ten days are up, but you can also sign a waiver saying that you do not need these ten days and you want your money now.
Of course, allowing people to waive their right to change their mind makes the whole thing pointless, since someone in desperation will always sign the waiver.