So this is the more expensive alternative. American Express traveler's checks can be cashed anywhere, but some AmEx offices cash them without a commission. Normally, the best rates for changing money are not found at the airport. Once settled, students exchange information about good places to change money. It is usually disadvantageous to change money at night or on weekends because the number of places open then limits the competition.
Many places offer exchange with no commission, but usually give less advantageous rates.
These are Plus Machines and possibly Cirrus. But check with your bank before you leave to obtain a listing of the International branches you can use for both Plus and Cirrus. If you use the STAR system, check with your bank for locations. If you have American Express or Thomas Cooke travelers checks you go to one of their offices in London. So if you have either American Express or Thomas Cooke travelers checks, you just pop in there for an exchange of your money into British Pounds.