Re: transfer the phone into Mum's name
I'm not sure how much it helps in death (probably not a great deal), but if you want to protect against your partner becoming incapacitated then you need to take out a "lasting power of attorney" before it happens or you'll be frozen out of your partner's finances until you go to court... and UK courts have quite a backlog.
But then again, even applying for LPA is a problem.
To be honest French bureaucracy sounds easier.