Your electricity meter can hold up to a total of £255, as can your electricity key. This means at any one time you can have a maximum of £510 on your meter and key.
Gas meters have no limit, so you can top up as much as you like.
For electricity meters, you can top up from £5, and with gas from as little as £1.
When topping up either a key or card, make sure the receipt says the credit was accepted and keep a hold of it, just in case you need it for future reference.