Thanks for the replies! I booked a bus 50min from arrival to be safe, since, even though it's offpeak, I'll have to go through the longer immigration line (international flight, not citizen of EU/EEA/CH).

I booked with National Express for this trip (GBP12 non-flexible), but I might try the Arriva bus next time I fly into LTN--having a flexible bus ticket seems really appealing.