Another way of looking at it is this:
IAH-LHR, practically everyone is going to be checking-in at T-24. No inbound BA conxs with earlier OLCIs. Greater initial selection, but options may suddenly decrease at T-24. Perhaps consider purchasing this one to secure the aisle on this overnight flight, especially if current options are minimal.
LHR-EDI, potentially lots of earlier OLCIs from across the network. It is only an 1.5 hour flight though…
EDI-LHR, very few advance OLCIs (maybe those doing back-to-backs).
LHR-IAH, medium advance OLCIs - those connecting in LHR from further east. A mixed result.
IME, also travelling with no status and only using OLCI, I've been able to choose reasonable seats at T-24, for myself and my wife, often getting bulkheads or aisles, etc., as desired. Sometimes it doesn't work and then I remind myself that someone has to sit in the middle, today it's my turn...