There has been opportunities over the years that Glastonbury could have got Biffy if they wanted them. 2014 for example, year off or not I imagine Biffy jumping at it. It was very much a time though when Glastonbury got the band before others did and you could almost guarantee a Glastonbury headliner went on to do Reading until recently times. Prince was also the reason Beyonce headlined Glastonbury 2011 instead of subbing Coldplay.
I don't think they were ever in the conversation for 2015 though and if they hadn't done it by then they weren't. They returned to sub in 2017 and chose to bump up Florence and get The Libs in on a quick in and out instead. A band who hadn't played for 4 years and then 6 years later still weren't given it and we know Simon Neil would bite the hand off for it.
That was a right day until Lil Uzi Vert (who was originally on the lineup) pulled out and it messed with the running order. It left Enter Shikari’s two sets clashing with The Story So Far and A Day To Remember. I remember cursing the clashfinder when I saw that! The Story So Far were amazing in the tent though and I had a blast crowd surfing for half their set. ADTR were the worst I’ve ever seen them that day. They looked so uninterested.