The London Air Ambulance is in fact partially funded by the NHS. They have an online donation form on their site:

http://www.londonsairambulance.org.uk/

About a dozen air ambulance accept donation through Just Giving, go to http://justgiving.com/ and search for "air ambulance".

P.S. Just Giving take a 5% transaction fee on every donation, but for small charities that is a more cost effective way of getting online donations than setting up their own system
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Remind me to change my signature to something more interesting someday