Temporary for the strike period.
LGW a bit short of a/c due to summer schedule and a couple of 734s out of service, so making use of some 'spare' 320s at LHR.
Also gives a little extra capacity (162 seats vs 147 on the 73) on flights which have been very busy due to the strike.
Most CC are 737/777/Airbus rated as a dual short haul fleet was the long term plan a few years back...!