I can only comment on Vodafone.
"broadstairs" has a quite detailed recent saga on FTTP BT/VF migration here, you may have seen this already.
https://forum.kitz.co.uk/index.php/topic,27887.msg469946.html#msg469946His experience prompted me to migrate my daughters line to VF, only been a few months but so far all good, the migration experience and provision of a new digital LL number was faultless.
I have now signed up for FTTP BB & LL migration to VF later this month, biggest factor for me being cost saving, quite substantial especially where digital LL provision is maintained.
My BB speed requirements are very modest by present day standards so the 200/27 Mbps package suited me.
The VF router is fairly basic but again sufficient for me, VF do permit use of 3rd party routers and will provide login details on request, this even extends to digital phone porting details if a LL is still required.
Can't comment on support, certainly has had bad reviews in the past, no reason to use as yet