You are only supposed to leave a bag in the cloakroom of the lounge while you are in the airport. However, as others have observed, would they realise? I had the same thought at the arrivals lounge, but figured there was less reason to leave, so more noticeable.

So I used the left luggage facility instead. Its £8 for the first 5 or so hours. Its at the far end where domestics arrive.