This subject has been through this 'cabin' many times. Firstly, the cost of issuing allocated seating and then monitoring it's execution is high - even if they start selling some aspects of it. FR sell 'priority boarding' that can be defeated if pax are bussed and BE sell seat allocation.
Secondly, and far more crucially, there are many UK pax that are intent on ignoring boarding sequence. Observe main line carriers at major airports, with allocated seating:- In the gate you can hear the call for "Rows 55 to 50" or "Boarding cards group A" or whatever, and observe a massive surge of 75% of pax towards the quaking agents.
You can then see that the agents have learnt it is better to allow them all through because of the verbal abuse they get if they try to make pax wait their turn. On board, the aisle is then blocked in the usual way with someone in Row 33 (say) preventing 50/100 people from getting to the back.
A very large reason for this happening, in my view, is NOT to bag a 'good' seat but to bag space for their bag in the lockers. Since people tend to bring too much with them (and are often not challenged about it for the same reason of verbal abuse) they now need to grab space for their bag so as to maximise their convenience. This is why they stop part way down the a/c in places where few people have taken seats - not for the seats but for the locker space. For those with allocated seating, they want to fill up the locker space before the other pax for that row arrive. We all know the problem of finding the locker full and having to go to locker either side to hope for space, thus compounding the problem.
Meanwhile, in the departure gate, the agents will continue to call out the rows/groups for boarding as per procedure - even though everyone knows that it is pointless.
This might sound cynical but paxing in the UK for 40 years (with LHR and LTN as my local fields) can have that effect on one.
