This is a once in a lifetime opportunity for UK to take back Hong Kong. Given the international tension right now, she has so much more leverage now comparing to the time when she signed the joint declaration.
There are costs but it also has the best return. After reclaiming Hong Kong, she can easily bring Hong Kong back as one of the fianical centre in Asia.
While there are many sectors infiltrated by CCP in Hong Kong, she can contain them here in Hong Kong without national security risk from infiltration from BN(O) scheme.
Given the Treaty of Nanking and the Convention of Peking, she has every right to own at least Hong Kong island and Kowloon.