We went abroad within 1st year, before AO. Firstly, you will need BF OR BM's birth certificate and one of their parents (your childs birth grandparent) birth certificate, if your child's BP is born after 1982. SS will then apply on your behalf. Only if they agree mind you, we had 5 SW's assess whether we could and 2 said no, but as 3 said yes we got them. So don't presume that even with documentation they will apply on your behalf.
I do agree with Italian. You need to think very carefully about whether your child will cope. I don't agree with a blanket rule of a certain time period because every child is different. Ours were absolutely fine (we're off on holiday again before even second year is complete) BUT we planned the hell out of it! Pictures of everything, room where we were staying, pics and videos of every place we were going to visit , pics of plane, airport etc. We had these all over a wall in our house. We watched every show on CBeebies that featured going on holiday. Wooly and Tig had a good one showing airport. We made sure luggage looked nothing like suitcases they moved to us with and we then made a big deal of decorating them with luggage tags etc. So we could find them at airport to bring them back to our house. We also had a family member drop us off at airport and pick us up who made a big deal about when we all return. We had count down calendar to going and when there, a countdown calendar to coming home. We bought gifts to bring back for people to reinforce going back and things to put in their bedrooms, again reinforcing they were going back to our house. We all slept in same room while away, we never left their sight.
It was a lot of effort but it paid off. We did have pre-schoolers so that worked well. If you have a much younger child who are pre-verbal they may be disturbed by change once again with no understanding of an explanation, so working around that will be impossible.
I do know of someone who went on holiday in this country and really struggled, the change was just too much for the child to handle and that's without flying for first time etc. I know we were lucky and are in the minority. So all I say is, go for it if you believe they can cope but plan, plan, plan, and try to think of everything that will scare them rather than everything that will excite them, leave the latter as the bonus when you get there :)