Joe, I shall try to explain myself a little more clearly.
If you are UK domiciled but resident overseas working for an overseas company, you are liable for neither tax nor NI in the UK. So if you carry a UK passport but work for CX in Hong Kong you will be exempt from UK tax and NI.
If you are domiciled in the UK, based in the UK, working for an offshore company, but resident in Estonia, you will pay neither tax nor NI in the UK.
If you are resident in the UK, working for an offshore employer you will pay tax but not NI
If you are domiciled UK, based in the UK, working for a UK based company but living overseas and spending less than 90 days in the UK per annum, you will not be liable to UK tax but will be liable for NI. Your employer will also be liable for NI. (Brown's team have now muddied the waters on the 90 day rule. It used to apply solely to the wage earner. A Tax Commissioner recently decided that the 90 days had to include your family and your home. So the boys working in Saudi on unaccompanied postings could find themselves having an interview with the revenue. I understand an appeal is to be mounted on this issue.)
The key to NI is where your employer is based. The key to tax is where you are resident. As I understand the current plan as it affects CX UK based crews, CX are going to remain offshore so they will not be liable for NI and neither will their employees, even the UK residents. If CX set up a UK based company then both CX and UK based employees will be liable for NI. As long as you are non-resident, you will not be liable for tax.
For the many UK based crews who are not UK domiciled, as long as they are non-resident they will not be liable to UK tax. I am not sure where they will stand on NI, although I suspect that, being non-domiciled, they will be exempt.
I hope you can find the category that applies to you.
As for HK tax, if you pay tax in your country of residence, you will not be liable for HK tax and you will be exempt from the twice yearly payment. This has to be arranged with the HK tax authorities. Alternatively you can continue to pay HK tax and offset it against any overseas liability.