I don’t think you’re supposed to leave bags there if you’re leaving the terminal but in reality they’ll not know.

However, due to conformance those at the security check will know you have left the terminal and so before reentering security you will need to go to a BA desk (so make sure you don’t leave your passport in your bag) to update the conformance record so you can re-enter landside.

Unless you want to spend a few hours in the CCR before heading to London it’s a lot easier to use the left luggage facility next to M&S in arrivals