Sadly I don't believe the dating app you wish for exists. There is a flip side to this, imagine being a Thai lady wanting to meet a nice farang to marry and using the dating apps, only to find mostly men looking for sex.
I think you can find a nice lady with a regular job on the dating sites, but you will need to talk to them and use some common sense.
you still need to prepare everything even if using an agent, so it's just as easy to do it yourself. I read a post by someone recently stating they had used an agent and they asked them to provide all sorts of nonsense (the agent clearly didn't know the process), obviously they were unsuccessful and just wasted their money. I would prefer to read the rules and prepare it myself so that I know it's correct. Using a Thai person for a UK process is a silly idea IMO, but if people want to waste money and fail that's up to them.
Whatever you do, don't pay a Thai visa agent to go through the process. They cannot bribe anybody and know as much or less about the process than you do. Read the guidelines and provide all of the evidence requested and more, also put it in a logical order with an index. Good luck.