I’m fully aware that an agent is not a lawyer, but l stand by my view that a lawyer isn’t necessary in all but the most complex of cases, which the OP’s isn’t.
Of course you are free to use a lawyer who may be expert in UK Immigration Law as well as other legal matters.
I’m not sure which sort of “l can do” agent you are referring to, but the agent recommended by TonyM is registered with the UK Regulatory Authority, even though that isn’t a legal requirement in Thailand, and highly experienced, as l say, the choice is of course yours.