I think the key question is whether you want to save £200 to fly in the middle of the night, rather than the middle of the day. PFO flights arrive at 0300 (Summer timetable), LCA flights arrive at 1630. For me that would ne the key factor.

If timings don't matter, then next consideration would be LHR or LGW.

Planes are tatty 767 vs tatty 737 (although possibly an A319) from LGW.

Seats are the same.

Food will be largely the same, but more professional serve IMHO from LHR (but do you really want to eat a Band 4 meal in the middle of the night?)

Personally I woudn't even consider a saving of £200 to fly at those ungodly flight times.