I think that your seats are pre-allocated on Flybe as well.
The point I was making was that Go used to pre-allocate seats in the days when most if not all low-cost airlines operating into and out of the UK did not, certainly not Ryanair and easyJet, although Debonair may have done so but I can't be sure - this was before Jet2 started operations and when Flybe was still Jersey European or it may have been in its British European guise by then, either way not a low-cost airline.
If Jet2 can pre-allocate and be as successful as it undoubtedly is, why can't the others?
easyJet, for example, operates its Speedy Boarder scheme which, in theory if not always in practice, gets you to the aircraft steps amongst the first to board. Why not just turn that into a specific seat because I'm sure they would want to add a premium if they began pre-allocating seats?
The main argument against seems to be that it would encourage passengers to dawdle in airport shops or bars instead of rushing to the aircraft to get a decent seat. Presumably Jet2 encounters no problems in this regard.