Mortage companies not liking cash basis accounting?

TCA

A limited company could not use a cash basis for accounting, so the OP is presumably a sole trader. As for it being a whole extra load of work for an accountant, that's largely rubbish unless there's a whole lot of transactions every month. Traditional accounting just means using accruals and prepayments, which is unlikely to mean more than a few items being recorded differently at the beginning and end of each accounting year.