The Travelex spread (for walk-up rates, for most currencies) is 12%-14%.

Cheapest way is to pay for everything on your plastic, because the spread is only 5.5% (2.75% each way). If you get a Nationwide credit card the spread is zero - saving you £27.50 (although no miles ....) for every £1,000 you charge.

Cheapest way to get cash is to use a cashpoint with your debit card on arrival. Even with a £1.50 charge from your bank for cash it is probably still cheaper with the 2.75% loading as long as you take enough out at once.

For Euro notes, the cheapest place in London is, I believe, the place near my office - BBVA (the Spanish? bank) on Cannon Street, opposite McDonalds. Only a 2.5% spread and no commission. BBVA also do US$ notes and TC's, but there is a commission charge with these.

To save hastle, take $100 in cash from Travelex and get the rest on your cash card. In the US, remember, it is normal to pay for everything - even at Starbucks - with plastic (in the UK most people seem to have a £5 mental threshold before the card comes out), and of couse you'll be getting miles as well.