I probably wouldn’t stop work with a million. It isn’t enough for me. Neither does OP, setting up a business:
Buy 3x properties for £200k and then live off the rents.
BTL as a single investment isn’t a great idea (eggs, baskets, etc).
If I had to quit work, some capital spends the invest tax efficiently. With risk you can drawdown 3.5-4% per year, indexed without affecting the capital so that’s a fair income for not working.
What I’d actually do is buy a more expensive house, couple of other things and then invest the rest. Then go back to work, able to retire significantly earlier than planned.