With the latter… choosing between flat rate and paid invoices depends on how smooth your work/cashflow is.
I chose the latter as my margin, relative to the turnover, varies quite wildly from job to job. Some jobs have a large proportion of spend on vat deductable goods, some I’m only really buying in labour, sometimes I’m contracted labour only so have a very low spend relative to turnover. So quarter to quarter my vat liability might be a few hundred quid of £10k.
Its a bit more work each quarter, but its work you’ve need to do eventually for your tax return anyway.