unitedabx, not true. You ARE entitled to a tax free allowance in the UK as a passport holder, even if you are non-resident. You can claim it by filling in form R43.
I'm unable to post the link but search R43 on the HMRC website and you'll find it, there's also a notes document there explaining it also.
I've been doing this for the last few years successfully, and as mentioned above would also recommend Acorah softwares Tax Calc.