I used the Visa4UK, same as you.
I always err on the side of caution and go OTT, on what they may require.
I always make sure that they include proof of money in bank, theirs and ours, proof of their properties (Full Chanotes not copies), proof of where they will be staying in the UK, proof (letters from my family and friends they have made on previous trips) that they will be visiting in the UK. I include Towns and Cities they will be visiting.
I did not translate English supporting evidence to Thai, but always had Thai documents translated (Certified and stamped) into English. I have some pics (head and shoulders) taken on their last visit, which I use for passport sized photos. I use a free, online site, to compose and print them off, then send them with their application forms to Thailand. Again, just in case.
A pain in the proverbial, but you can bet that any document they ask for in English, is the one that you have not had translated.
I also always get them to take colour copies of their passport pages, old and new and copies of anything else that you submit with your application, such as expired passports, just in case you meet an interviewer with a hangover who wants to make life difficult.
The only problem I ever encountered was, as I mentioned, getting a block booking for the interviews.
My wife was in BKK for one application I was making for them and I had told her that I was finding it very difficult getting them all in on the same day.
She went to see a certain company, who I won't name, who, I believe, submit the applications to BKK Imm. They said, for a fee of Baht 16,000 each application
they would walk them across the road and get them all in together.
Needless to say, the wife put her straight on that little scam.
Eventually, I managed to get a block booking online, but had to stay up until the early hours on several nights, to wait for people to cancel their appointments, then grab them, when 4 slots on the same day came free.
Hence the application for 10 year visas.
All turned out fine and they all received their passports back, complete with the visa stamp, within 7 days by post.
Wrap your application up in a nice ring binder and you will be fine.