One point to make - most of the flights to SFO leave in the morning.

Therefore you have to spend the night in London before returning home as there's no chance that you could fly from Barcelona to Luton then transfer to Heathrow in time for your flight.

Have you factored in those costs?

Additionally if you are non-connecting tickets and you miss your second flight, none of the airlines involved will lift a finger to help you - except to sell you some very expensive one way tickets which for 5 people will be a pretty penny

Either drop Barcelona, revise your plans to allow a couple of days in London at either end of your trip or - buy a SFO to LON & BCN to SFO open jaw ticket and book LON to BCN separately