Are we just talking about two different things? No entry vs no travel (ie can't be checked in to travel)?
Currently, enforcement of the UK ETA is up to airlines. It is done manually by airline staff asking passengers if they have one. The ETA is not enforced electronically.
If you are required to have an ETA, but manage to travel to the UK without getting one, most likely you will have no trouble entering the UK.
When you apply for an ETA some electronic checks are run on your passport details. At the moment, when you insert your passport into an e-gate to enter the UK (or present it to a UKBF officer), essentially the same electronic checks are performed.
So if your ETA application would have been refused, you won't get into the UK anyway, but if it would have been granted, there isn't really any problem (apart from the fact that you ignored the rules).
From 25 February, presumably the ETA system will be linked to airline check-in systems and the e-gates, so that without an ETA when you were required to have one, you won't be allowed to check in. If you somehow manage to travel to the UK anyway, the airline may be penalised.
If you are on an itinerary involving LHR airside transit from and to airports outside the Common Travel Area, you still won't need an ETA, but if you try to enter the UK during your transit, presumably the e-gates will reject you and tell you to apply on the spot.
www.gov.uk
