I would recommend withdrawing money from an ATM when you arrive. There is an ATM in the arrivals hall.
I have found that you get the best exchange rates from ATM machines.
If you get a Nationwide account, they are the only bank which won't charge for foreign currency withdrawls, and yet still have a very competitive exchange rate.
I recently went round the world and followed this approach.
If you are not happy with this, I would get some cash, but be careful if you are taking large quantities. Do not use the Post Office for foreign currency, as the exchange rate is awful. If you book the foreign currency exchange in advance at Travelex at the airport you get a good rate, if you turn up at Travelex when you get to the airport, you get a different exchange rate, which is awful.