Essentially I think it is different policies. LGW also owns EDI and EDI also offers pre-check-in. There is a specific point about LGW - in the past they used to have a huge number of passengers checking in between 07:00 and 12:00 particularly in the summer and ski peaks. By getting 10% (?) of the your workload dealt with during a quiet period is probably an all round win-win. As it happens T5 (but not T3) also have relatively quiet evenings so the usual constraint - insufficient storage space - would not apply, I would have thought. On the other hand LHR has much more automation than LGW so peaks are less of a factor.