Citizen Advice Bureau.
They can advise on whether the installer broke the law with respect to the installation and what he has done to tie you to his services if indeed he has.
If the engineer has locked your panel getting another engineer out would probably tell you the panel needs replacing.
However you using it for 10 years would suggest that the service timer which you say has been active since day 1 hasn't locked you out?
Some panels have different timers, 1st gives you the warning when the time expires, 2nd every time you arm and disarm, and the worst kind which is the panel locks out completely