Most airlines won't bat an eyelid if you are back home before you are 27 weeks. Check on their website. After that, they might need a doc's certificate - and if you look quite big but are within 27 weeks make sure you have your notes to hand so you can prove at check-in that you are OK to fly without doc's certificate in case they do get jittery. This happened to me on way back from hols to Oman before DD1 was born - I was 26 weeks, no comment on the way out but in Muscat they started saying I might not be able to fly. Duh. Like I'm going to be less pregnant tomorrow...They treated me like a total inconvenience and a risk rather than a fertility goddess, which threw me slightly!
Anyway, also check your travel insurance - remarkably, many travel insurers have completely different terms to the airlines, so you need to check you are covered under your current policy. If not, go to the Post Office - they are fab. And of course, stay hydrated and wear flight socks. And have a fantastic pre-baby break!