If you board the aircraft in London the check in agents know which countries are visa exempt. You are absolutely wrong this would cause a problem. Either way you will need an onward travel out of Thailand. This could be a problem if Thai immigration ask you for proof of onward travel.
When I got a new Belgium passport in Bangkok the embassy gave me a letter explaining why I got a new passport. With this letter and my old and new passports Thai immigration transferred the necessary information.
If you do not have a motorcycle licence from your country go to a driving school like Mityon. The course can be done in 6 days and they do all the testing. Just take the paperwork to Land and Transfer office. Course is not expensive
You should find a Visa Agent and they will look after everything for you. It is more expensive than doing it yourself but everything is looked after. It will cost you up to 30,000 THB. Following years do it yourself.