A lot of cost of cylinder testing goes into postage
Ideally use HPAC - at about £30 handed over personnaly
Ask your local site, or an event that you attend, then just hand it over and they will get it to HPAC or a tester
(HPAC will probably be providing the air at many events)
Then pick it up at a later date
Failing the above check out local dive shops, which can also arrange testing. Cut out the middle man by checking the idest list to find out the places that dive shops will use to do the test
https://www.sita.org.uk/idest/idest_members.php