If its your first trip, you'll get confused by all the coins. 1p 2p 5p 20p 50p.
Keep everything below 20p coins in a jar and give them away. Your pants pockets will thank you, and you won't be fumbling around at a checkout stand trying to figure out what's what.
In 2008, we bought some British pounds at a bank in Seattle. When we were at the Chelsea Flower Show, none of the vendors would take our money. The US bank had given us an old series of notes. We had to go to a local bank and exchange them for new notes. Now, we follow Rick's advice, and just use an ATM when we arrive.