I’m sorry for jumping in this conversation (I’m not in UK), but I have the same problem with Vodafone CZ. No Wifi calling settings. Wifi calling top menu icon greyed out.
However, now I can confirm that when I switch to airplane and enable wifi, I can actually receive SMS and make phone calls.
I’ve found one way to check whether this “hidden wifi calling mode” is active.
If you draw from the top to display settings, you don’t know anything:
However, if you swipe down once more to show the two-row quick settings, the network name is actually there in the upper right corner!
When this mode is active, you also get the wifi calling icon next to your contacts in the phone app:
If you have a decent linux router at home through which your phone is connected, you can also try running this command on it to see whether some Wifi calling traffic is actually flowing through it:
tcpdump -i YOUR_ROUTERS_BRIDGE_INTERFACE esp or udp port 4500 or udp port 500
It should show a burst of packets as soon as you switch to airplane and connect the phone to the wifi. If not, then your wifi calling is doomed 
This is how the first few lines look for me:
02:31:40.041010 IP 192.168.1.186.42074 > epdg.epc.mnc003.mcc230.pub.3gppnetwork.org.500: isakmp: parent_sa ikev2_init[I]
02:31:40.056339 IP epdg.epc.mnc003.mcc230.pub.3gppnetwork.org.500 > 192.168.1.186.42074: isakmp: parent_sa ikev2_init[R]
02:31:40.119342 IP 192.168.1.186.42074 > epdg.epc.mnc003.mcc230.pub.3gppnetwork.org.4500: NONESP-encap: isakmp: child_sa ikev2_auth[I]
02:31:40.285068 IP epdg.epc.mnc003.mcc230.pub.3gppnetwork.org.4500 > 192.168.1.186.42074: NONESP-encap: isakmp: child_sa ikev2_auth[R]
What’s interesting is that when I’m in this “hidden wifi calling mode”, IMS service status (call *#* #4636 #*#*, choose IMS status from the three-dot menu) says registered. But when I switch off airplane mode, it says not registered. Now this is something I’m gonna #contactsupport about.
IMS service status in the hidden wifi calling mode:
IMS service status when connected via normal mobile network:
What’s good is that if I switch the phone to airplane and reboot, the phone does not need to connect the mobile service to be able to go into this hidden wifi calling mode. Not even once. So the provisioning from the network is probably only one-time and then it works always.
I’m not sure, however, if the operator does not want to somehow register each Wifi calling-capable router/site via the mobile network. That could mean that places which have no reception could not have wifi calling enabled. But that’s pure speculation from my side. A crazy idea could be to take your home router, take it someplace with reception and connect an Ethernet cable with a quality link to the internet