Hi killerman
Quite often your bank will do this for you free of charge, depending on the type of account you have with them. It is worth asking them first.
The Post Office is also very good as is M&S
Have nice holiday
travelex online are very good and you can arrange to pick them up at the airport on your day of departure,saves carrying them around,until you check in.
M&S are the best place on the High Street but if you are in London there are several better - eg Thomas Global. You could also try a cheque cashing shop - some of these have expanded into the holiday money business and give good rates.
Travelex are good - providing that the debit card you pay with doesn't charge a fee. If it does it wipes out the gain you made.
A debit card in an ATM in Florida is usually better still provided you don't make frequent small withdrawals.
Spend it all while you are there - you'll have an unpleasant surprise changing it back otherwise.