Is this something that I can do myself, here in the UK, on behalf of my GF ? I don't see it mentioned anywhere on the website (maybe I've missed it).

It would make things a lot simpler if I could pay the fee and complete the application online for her.

Yes you could, and the application fee has to be paid online anyway.

It does remain your girlfriends application though, she will still need to submit a hard copy of the application together will all her supporting evidence, and of course her passport, at the Visa Application Centre in Bangkok.

https://www.visa4uk.fco.gov.uk/#

http://www.vfs-uk-th.com/

In many ways it is no different to being here. You would submit online and your GF attends the interview.

The only disadvantage you have is that you are not here to 'package' the application for her.

That is quite important in my opinion as the main task is to make the ECO's job as easy as possible and ensure they have no reason to say no.

Thanks for the confirmation. Yes I know it's her application but I'll be doing most of the document gathering, and packaging/presentation before mailing it to her, to take to the visa application centre. I'll get her to write a covering letter (briefed by me) explaining that she's had my help in preparing the application.

On the subject of the application questions, one of them asks about her previous passport (start and end date, etc), but she recently got a new passport as a replacement because she couldn't find her old one. I'm wondering what to put down here - is it okay to tell them she lost her last passport (will that sound dodgy or am I just being paranoid?). I've told her to try and find out the details of her old passport, by phoning the passport office but I'm not sure they'll be able to help.

No you're not being paranoid, the ECO may well be concerned it the passport isn't available as it could contain negative immigration history, as well as positive.

She needs to indicate the passport was lost, if she has a police report then include a copy of that, but in any case try and include any details that the passport may contain in the comments section as well as her covering letter.

Non production of a passport may make the ECO suspicious, she just needs to allay those suspicions. The loss of a previous passport in itself will not cause an application to fail.

She didn't report it to police because it wasn't stolen, she's just misplaced it. It could be in any one of 4 or 5 addresses she's lived at over the last 10 years. I can see how its easily lost - it's something she's never used - she's never travelled outside Thailand, so there's no immigration history. Obviously I don't want to give the ECO any encouragement to reject the application, but I don't see anything else that she can do except say it was lost. She has a valid current passport, that should be the main thing.