AC, here's some info from a 2 min Wiki search
"After the Tiananmen Square protests of 1989, the Executive Councillors and the Legislative Councillors of Hong Kong unexpectedly held an urgent meeting, in which they agreed unanimously that the British Government should give the people of Hong Kong the right of abode in the United Kingdom.
More than 10,000 Hong Kong residents rushed to Central in order to get an application form for residency in the United Kingdom. On the eve of the deadline, over 100,000 lined up overnight for a British National (Overseas) application form. While mass migration began well before 1989, the event led to the peak migration year in 1992 with 66,000 leaving. "
So yes, right to relocate was allowed.
As for the executive law implemented, yes that was british but it hadnt been used for over 60 years. It should have been repealed definitely, but I guess it had been forgotten since it hadnt been used for so long.