We don't yet have a large enough sample size of cases like this to study, from which we could draw conclusions about the deterrent effect of more severe sentences. (At the time of writing, n=0, as the perps - if any - have yet to be found or motive etc established.)
To pick some groups who've been speculated about - jihadists, eco-whatevers, engineering students out looking for lulz, highly sophisticated criminal gangs pulling off Italian Jobs, sinister politicians conspiring with MI5 and persons confused about the best way to express their opinions about Brexit -- all have a different propensity to think again in response to another year on their possible sentence.
Arguably, as there's no precedent for the purported attack, there's no deterrent at all. There are no previous perps they could look up and think "Hey, this guy shutdown Stansted for 18 hours and got 12 weeks and 1000 hours community service!" or "...had his goollies cut off"* , as the case may hypothetically have been.
* With apologies to Jones, Smith and Stephenson https://youtu.be/p6aQC-1-GF4