S'all very strange.
I had a challenge with my Chevy Blazer. Story goes,
When Chevrolet imported the truck in 1999 the ABI (Association of British Insurers) apparently insisted on THATCHAM 1 alarm. Chevy therdefore fitted the THATCHAM 1 alarm alongside the US alarm, as they arrived at Soton docks.
I had the car for 9 years plus and then had electonic immobiliser issues. 3 recoveries by low loader. All dealt with by a UK breakdown company. Conclusion, car worked fine if you bypassed the UK immobiliser, apparently 1 piece of wire sorted that out.
It turned out that the UK THATCHAM immobiliser had failed, with no replacement parts available. US systen still working fine.
However, UK insurer had issues with the lack of UK system
So had to insure as not having THATCHAM 1 system.
Makes sense, no, did I have to live with it, yes!
It's a pain but shop around and decide on what is acceptable to you. Underwriters will decise on individual cases so be clear and know what you are prepared to pay.
All part of this great hobby!!!!!!!!!!