"I suspect you split your invoices after deducting VAT."
Yes almost certainly - although that only added 13% due to the cash services scheme*
The other option is that I am misremembering and I actually did the split 60/40, that's also possible - it's . a few years back.
* Smaller VAT registered businesses could charge the full 17.5%, abut only give 13% to HMRC - in return you didn't get to claim back VAT on purchases...
Since what I was selling was basically my time it was economically sensible for me to not bother accounting fir VAT on expenses, but just to pay it - even ignoring the added time for the accounting and VAT returns - and get a bit of a kick back on the charged VAT.