I've had good experience like that with Jet2 too carabos. As in my previous post, I almost never pay for seats. The only exception is with Ryanair, where you can't check in your return flight if it is a week away or more without paying.
To me, £12 is a small price to pay to avoid having to fiddle about checking in on holiday, finding somewhere to print, or wifi to do it on your phone and then not break your phone or have flat battery on the way back. I like to switch off from admin stuff and internet on holiday and just want to get it all sorted before I go. I don't like paying it but that's my choice.
Anyway, Jet2 - when checking in online they often seat me and DP separately, but more than once when we got to the airport, they offered to change for free at the bag drop, so we were together. I don't mind either way. We are grown ups who can exist separately for a couple of hours. I usually sleep or read and DP usually watches films or falls asleep and snores anyway.
The planes are usually full or very nearly full, but they somehow must have been able to do this without upsetting anyone else?
Another time, they offered us the extra legroom seats for £15 for the pair of us, which I'm sure was a lot less than when checking in online so we took them, this was well worth it as we were going to one of the Canary islands, so a 4.5 hour flight.
Jet2 are usually a little more expensive than Ryanair, but you do get more luggage allowance and the flights are often at more civilised times.