I noticed that that link confirms my position (I'm the only person in this chat pointing this out so I will emphasise it again): you must submit your old passport and they will send it to the UK with the rest of the documents. You do not get to keep it.
I suspect most people in this chat are either not UK citizens, or have never applied for a new passport from Thailand. To apply for a new passport from Thailand you must prove that you are resident in Thailand. You cannot be a mere tourist by your visa.
To quote the link:
Do I need to send original documents with my application?
Yes, you must send your old passport, and in some cases, additional supporting documents (such as a name change certificate).