Hmm. I've certainly been at 110 on a GWR 387 a few times.
It doesn't have to be the ECML, it may well be something like the Kings Lynn section. If the pantograph has to be replaced with a different design to cope with issues there, this may prevent 110mph everywhere. Others will no doubt be able to elaborate further, but I believe this is, or certainly was, the issue.
In reality, 110mph will probably not be of too much immediate benefit on GN. The current timetable is based on 100mph. The higher speed doesn't start until some way out of King's Cross, and the chances of catching up with something else is quite high. Adjusting the timings may well require more extensive timetable changes, plus it would then introduce an issue should 365s stand in, which is still quite possible particularly on services to Peterborough Cambridge and Ely although less likely beyond due to the dividing/joining at Cambridge.
I'm sure 110mph will come eventually, but I can't see a pressing need for it especially with the current timetable.
GW is a different route with different infrastructure and different issues.
Don't forget GN was originally going to get the 100mph 377/5s, so evidently 110 wasn't seen as essential.