If your puppy chews up a £20 note or you wash and tumble dry one left in your pocket, your bank should redeem it if it's in fairly good shape. Even if it's badly mauled, take all the scraps along to your bank. If the note can be identified (if the bits you have show the number) or if you have more than half of the note, the bank will probably replace it immediately. You will have to sign a declaration to say what happened.
If a wad of notes gets burned or mutilated, wrap the remains in cotton or polystyrene chips so that they don't disintegrate further, and take them to your bank. They will send them to the Bank of England, where specialists will try to identify the numbers. They will test the paper and look for the silver strips.
If you have a few corroded or charred coins, take them to a bank for redemption. The bank will send them to the mint for assessment and cleaning.