I normally go by how willing they are to let you take it home to repair etc.
They should all let you take the car away for repairs. Thing is really, the days of full free retests are long gone. There are only a few faults where the test is a partial test (usualy only if returned within 24 hours anyway) otherwise the garage has to to a full MOT. As they all now buy MOT "slots" from VOSA and can only do so many tests per day they would have to lose out on charging somone for a full MOT. Hence they are unlikely to do your retest for free.
If every customer went in for an MOT failed, took the car home and fixed themselves and then returned for a free retest the test stations would go bust rapidly! As far as I am aware the VOSA recommended charge for MOT on a car is £44 ish although £35 seems about the going rate around here...