There have been reports online lately that foreigners teaching here legally who have reached the age of 60 have not been able to extend their work RP's.
In addition, other reports on the net have highlighted that some foreign teachers who have taught legally for uninterrupted spells of five years here, have had extensions to their work RP's refused, and informed that they must return to their home countries to spend at least two years there if they wish to restart their teaching careers here.
It looks like the Foreign Affairs Bureau in Beijing is now applying considerable pressure to all provincial PSB offices to follow the two aforementioned previously dormant restrictive regulations to the letter. Time will tell if this is indeed the case.
If I were in your shoes, I would take the above potential problems into account before coming to a decision re the best way to proceed regarding UK tax avoidance by residing here.
Changing nationality is surprisingly very difficult, and not an option in the eyes of many a foreigner, rightly so in my view!
Marrying a Chinese national would give you an extra option, but that is a huge step with its own disadvantages as well as just one or two legal advantage, long-term renewable L visas being the less complicated one to take advantage of. But love is love, you never know whom you might happen to meet!