Double Taxation / UK Tax Return

I live in Croatia but rent out my house in UK, so I have to do UK tax returns. My understanding of double taxation was that I could get my personal allowance the same as if I lived in UK because a double taxation treaty exists between UK and Croatia. I don't know if this has changed but this year there was a question on the return that I've not previously completed. I file returns online and it seems I need an additional claim form for the double taxation details. Actually this year for some reason I received a paper form but there was no way I could have got it back to UK by the deadline of October 31 as it only arrived about a week before. It did not include this HS304 claim form in any case. Has anyone here had to deal with this issue?