Buying currency at the airport vs the High Street

Hi all,
Need to get some US dollars for the trip next week. Normally I just buy an amount at the airport but I've never really stopped to consider what this is like for value-for-money. Are there any noticeable differences between the buy/sell rates you get at Natwest et al, and the kiosks to be found in LHR T3?
Cheers,
Darryl
Need to get some US dollars for the trip next week. Normally I just buy an amount at the airport but I've never really stopped to consider what this is like for value-for-money. Are there any noticeable differences between the buy/sell rates you get at Natwest et al, and the kiosks to be found in LHR T3?
Cheers,
Darryl