As it also appears does the 4 weekly LHR-LPA service - reverting back to the once week BA A319 on Saturdays

Were they just 'slot holding' routes?

Yes, and they were operated by IBX as the slot timings would have required BA to night stop an aircraft away from LHR which they don't do for the LHR leisure routes.

Maybe BA will commence LGW to LPA as it's a missing link in there leisure destinations served from the airport.

They previously announced a 4 x weekly LGW-LPA but then switched passengers onto the IBX service. We'll see if it does get announced now the IBX appears to be cancelled.