Just to add to that, I recently did a bit of testing on BT wifi. Firstly, if the hotspot doesn't appear on the My Vodafone app (where you need to turn automatic connection on as it's off by default) or the coverage checker, you can't use it. Only premium hotspots are available.
Any BT hotspot will offer the drop-down menu, which includes a Vodafone option and implies you can use it), but this seems to be a BT default and is nothing to do with Vodafone. The problem is that BT/Openzone wifi comes in various flavours, but they all have the same branding, so what's actually available is about as clear as mud. As the hostpots you can use are premium ones, I also suspect they're public anyway (eg Starbucks), so all the My Vodafone app offes you is the convenience of seamless connection. I'm also not sure whether, on those, the usage limit included in your contract applies.
Available BT hotspots are so few and far between, and rarely in convenient places, that I don't bother with them and just use a free account from The Cloud and their Fastconnect app. They have more sites and they also seem to have better range.
