Its pretty common to se the AC systems on the top of roofs on big buildings in the US. From what I read, exposing the AC condenser unit to the sun should have minimal impact. Air flow through the condenser unit matters the most. Perhaps they were undersized for the extreme heat now happening in the UK.

AC systems don't quite care about direct sunlight, they are forced air heat exchangers. Now if the condenser (rooftop) side is undersized and therefore inadequate given elevated ambient air temperature all you can do is shut down a portion of evaporation side (the cold one) off, or the entire system just stops working.

Alternatively one can install water sprinkers on roofs like they do in China.