What tests, if any, are performed to check microphone quality and performance in the Uk ?
None, as long as it works and Tx/Rx is 5x5 then no other checks are done. The radios on the other hand are maintained and checked (along with all other ATC equipment) as per CAP670.
Airlite 100s seem to be the most common sets, comfy on the head/ears and good sound but not the most durable.