This is the inconvenient truth. The UK government signed an internationally binding treaty but did not put in sanctioning mechanisms for when the agreement is breached. SNP leader Jim Sillars actually warned Thatcher that the 1984 Sino-British JD would be unenforceable /1 https://twitter.com/oiliveHK/status/1266014388391047174

"The UK government was warned in 1989 that after the handover it would be 'impotent' and 'without remedy' if China breached the then-recently-signed Joint Declaration on Hong Kong’s future, declassified documents have shown." /2 https://hongkongfp.com/2017/07/22/declassified-files-uk-warned-1989-impotent-hong-kong-deal-breached-handover/

But let me be very clear: Thatcher's error of judgment doesn't mean that the current British government is off the hook. In concert with the US the UK government should incur a heavy penalty for the Chinese Communist Party violating the 1984 Sino-British Joint Declaration /End