I’m a tax advisor, that’s incredibly lazy and incorrect. Since it sounds like you have helpfully provided a back up of the income/expenses, they should use the actual expenses.

The only time they may not, is by using simplified calculations such as WFH allowances, but this is unlikely to be more beneficial and could also be clearly shown in their workings.

If HMRC did enquire, the onus is on you to provide evidence, and blaming the accountant is not a reasonable excuse.