Given that you appear to be paying UK tax on your salary that would suggest that you did not complete a P85
at the time that you left the UK. If you did, I'd be interested to know what response you got if any from the tax office in the UK.
Unless you are treated as resident in another country it can be difficult though not impossible to lose your UK residency status. I would suggest that if you have not completed a P85 before now, that you do so and send it to the tax office with a covering letter setting out your last 5 years. Regardless of your tax residency status in the UK, you will still be liable to UK tax on your UK rental income.
Having said that it is difficult to be treated as non-UK resident if you don't have official residency elsewhere (though you are clearly living in China), you should look at the new tax residency rules set out in RDR3 here
. A more readily understandable summary of the rules can be found here
. These became effective from 6 April 2013. I would say that you pass one or more of the automatic overseas tests so it may be worthwhile making the claim for non-UK tax residency.
Article 4 of the UK/China tax treaty here
says that you will not be treated as resident in China if you are only taxable in China on Chinese sources of income and capital gains. If you were treated as both UK and China resident at the same time, Article 4 allows the individual to ask for an adjudication by the relevant tax authorities of both countries.
Article 15 of the tax treaty deals with employment income. Give that you spend all your time in China and you work for a Chinese employer, your earnings will be taxable there. What needs to be decided is whether you should be paying tax in the UK on the China earnings. If you should, you should get credit for the Chinese tax paid.
Given that you are not benefiting from UK government services that you are contributing towards, if I were you I'd be inclined to complete and submit a P85 to the UK tax office and await a response. Any national insurance you pay, however, even voluntary contributions will help towards benefit entitlements. There will be nothing to stop you returning to the UK in the future.
I hope this helps but let me know if you have any further questions.