I wouldn't read too much into this - Amazon's Ireland region was deployed earlier (2008?) than London (2016?) and seems to receive updates earlier too.
London only came online relatively recently, maybe there's some operational stuff getting in the way of deploying? Or perhaps London has relatively few users at the moment, so the number of clients who will be able to take advantage of more specialised instances is also relatively low?
The London Region is really new. Amazon are using it as a way in to UK-only projects (specifically healthcare due to NHS regulations). I suspect their datacentres are considerably smaller than that of Ireland along with their client-base for the moment.
I wouldn't read too much in to it, brexit-wise.