I just googled and found this thread which seems to confirm that: How does Freetrade make money? and Freetrade Review - can you trade stocks for free? - Money To The Masses
I didn’t realise it was illegal for them in the UK to receive money from the brokers for giving them business, TBH I don’t see why they shouldn’t be entitled to make commercial agreements for x% of the profit from the spread, as long as the spread is the same as for every customer.
But anyway, seems you’re right.