The advice about Advance tickets on the return journey is very good, and indeed I avoid them, especially as Super Off Peak singles on GWR are reasonably priced.
However, I do note that TransPennine allow Advance tickets from Manchester Airport to be used up to 3 hours after the booked time, which I think is a really good idea, and I wish it was replicated by other TOCs serving airport stations. (Note you do need to get your ticket endorsed at the airport station before departure).
If your flight is delayed by more than 3 hours, then compensation usually* starts to kick in, so the cost of a new ticket can reasonably be covered by that.
*Unless your inbound flight is not with a UK/EU airline.