Have you looked him up on CoH, OP? Because I would
And being an accountant, you'd know as well as I do that a solid business name and reputation is no guarantee against blatant fraud. In fact, it makes getting away with it more possible.
Asking for cash is not breaking any laws Not in itself, no.
But the OP says that the trader is insisting on it and refusing to take the job unless payment is in cash, which is very unusual for such a large contract in these days of instant online bank transfers.
In any case, cash payments that large would be subject to money laundering regulations, and the OP drawing that much out of their bank account would ring all sorts of alarm bells with the bank.
To my mind, the trader would not only be evading VAT, he would very likely be 'forgetting' to declare his earnings for income tax as well. Someone with 5 or 6 jobs that size a year would by law have to register for VAT anyway, as they would be over the threshold.
So in this particular case, I'd say that the trader asking for payment in this much cash is as dodgy as fuck.