Have you searched google for Dive Test Centres or Scuba test centres or Diving equipment test centres.

I know there is a national network and that the centres need to be affiliated / authorised by the organisation. They produce a list of authorised test centres and the services they can offer.

I went to a place not for from the Stansted junction on the M11 last time ... Planet Scuba Raynham Rd, Bishops Stortford.
I need the next test in Aug 2022 but am not sure they are still there??/

Cheers, Phil