VoWifi not UMA
UMA was a way of bridging voice calls to the traditional circuit switched core via a UMA Controller, which pretended to be a BSC or RNC as far as the core network was concerned.
Modern VoWifi works differently by creating an IPSec tunnel from the phone, over Wifi, across the Internet to an ePDG on the edge of the operator's packet core where it gets converted to a GTP-U tunnel to connect to the GGSN or PDN-GW...and from there connects to the IMS just like a VoLTE service does.
So it's really VoLTE diverted over Wifi
Handover between cellular and Wifi can be provided by SRVCC (Single Radio Voice Call Continuity) if the ePDG supports it.
Although Vodafone might be using good old UMA and routing traffic via their CS core, I'd be amazed if they were