As has been mentioned, you can turn on Airplane Mode on your iPhone, but keep wifi (and bluetooth) on while doing so.
Usually, the first time you activate Airplane Mode, your phone will disable cellular, wifi, and bluetooth. It will keep doing that by default, unless and until you choose to turn wifi and/or bluetooth on individually while in Airplane Mode. The iPhone then will learn that preference, and keep wifi and/or bluetooth on even when you activate Airplane Mode because it thinks that's what you prefer. This is handy for situations like yours, where you still want to make wifi enabled calls through third party apps (or use bluetooth headphones, or even connect to wifi on an airplane, or any other scenario where someone will want to disable the cellular functions, but keep other data transfer services active). Airplane Mode will always disable the cellular service, though, as that's really the defining characteristic of Airplane Mode.
Even if you activate airplane mode, then manually enable wifi, I'd still suggest going in to your Settings > Cellular > then turn off Cellular Data and turn off Data Roaming.
With the above settings, and no data plan, the iPhone might as well just be no more than a wifi access device - like an iPad without a data plan, or a Kindle or laptop computer. You can then use WhatsApp on wifi only. The signal on WhatsApp will be no different from a Facetime signal - the wifi signal coming in to the phone will be the same for all apps and functions as signal strength applies to the whole device, and not individual apps on it.
For extra peace of mind, you can set up your phone to test out WhatsApp (and your Airplane Mode, and cellular settings) at home in Canada before traveling. Once you have cellular turned off and are using it with wifi only, it will perform the same way in your living room in Canada as it would in a B&B in Wales - there's no practical difference at all. That way, you can get familiar with the settings, fiddle around with WhatsApp and test a couple calls/texts, and be sure you're all set and ready to go! Even when your daughter installs the app and sets it up for you, you guys can test everything out and be sure it's working how you want before your trip.
Have fun on your trip!! :)