In the USA at least, lenders are real picky about loans like that.
They made a decision about their loan based on what they knew of your indebtedness. You suddenly adding a bunch more as a bridge, or a down payment source or whatever, at the last minute is a Big Deal. As in a showstopper. Even if the loan comes from the “Bank of Mom & Dad” and is only squishily meant to be repaid.
The main mortgage lender(s) desperately wants you the debtor to have made a significant down payment with your own funds so you have skin in the game from the git-go. There certainly are no-down required loans out there for special circumstances. But they’re not the default. At least not in the USA.
The solicitor where you are in the UK may have been concerned about some similar aspect, rather than that the source of funds was criminal.