Hello, I'm Keith and happy to help you with your question.
Firstly things will go much smoother if, when you leave the UK, you send a form P85 to your tax office. HMRC will then record you as non-resident for the following tax year and split the year into two portions, one resident and one non resident. When you return to the UK you advise your tax office and the situation is reversed. However, you are correct in your thoughts that if you do return as you surmise, for tax purposes, you will never become non-resident. However, if you do use the P85 process and do stay away longer than anticipated all will be in order for your tax affairs. You need to be away for a full tax, not calender, year to achieve non-resident status and the legislation takes that back to the departure date. I suspect that you will probably not achieve this and be treated for tax purposes as resident for the whole period abroad. This means that all your world wide income in the period will be subject to UK taxation, but for most countries there are Double Tax Treaties in place which allow tax deducted by overseas governments to be allowed as tax credits against UK liabilities.
Living at your girl friend's residence does not constitute you having a home in the UK for tax purposes, despite you being classed for tax purposes as resident. Paying her mortgage is merely a gift of moneys to her and gifts are outside the scope of UK tax legislation.
I do hope I have thrown some light on your tax position. Please don't forget the P85 procedure, you can file it on line; it will make life so much easier. A colleague has reminded me that the departure year split is now enshrined in legislation; it used to be an extra statutory concession. The effect is the same!