Gipilot:
Do not get confused here: a double tax agreement with your country and hong kong is irrelevant, you will be employed by a BRITISH company, that is what the fuss is all about. this british company deducts british tax upfront from your salary , plus NHS and god knows what more.
The reason why they offered you only a manchester base is exactly because of that, eveybody who could fled to other bases, these are now full with a huge waiting line ( problem is these bases are next up for a similar change). I am not telling you not to accept the job, that is your personal decision, but you might want to check with a tax lawyer in your country asap, you WILL end up with much less money than you expect.