To be honest I don't mind doctors charging for letters which aren't part of the NHS service. If you consider the amount of times a GP is asked to write a letter, for various reasons multiplied by the amount of patients the practice covers, it could take up a fair bit lot of their time and resources. GPs are being inundated with requests for letters from people making benefit claims, work medicals etc etc., as well.
The NHS provides a health service free at point of use for everyone and I don't think we fully appreciate how lucky we are to have it, and it's services are oversubscribed as it is . If I was going abroad and needed this letter I'd just view it as one of the extra but necessary costs like travel insurance , or fuel duty , airport charges etc. if you ask any other professional to provide a service for you, you'll pay for it. So why object to paying for a service which you're requesting that isn't part of your GPs remit or a basic health need?
The prospect of paying a fee each time you visit the GP is a somewhat different discussion, but it's being floated as a possibility due to the fact the NHS is nearly on its knees financially now.