I don't think you can blame having to say the word hectopascal on a read back to ATC (which takes less than 1 second) is the cause for delays at Luton ... I mean, really???
Yes pushing back blocks taxiways, but have you seen the cul-de-sac at Manchester - this is not a specific problem, and from what I have seen of LTN they do try to plan stands and plan ground movements in such a way to minimise these delays.
I think a lot of the problems come from having an airport which is more leisure focused and hence the passenger mix tend to (on average, and I know this not not exclusive), but on average passengers tend to travel less, are therefore less airport savvy and inadvertently cause delays ... it's the nature of leisure orientated low cost travel