For around £170 the BT system sounds like an ideal solution.
We have a hard wired ethernet network in our large stone built house, so haven't experienced any such issues or the need to try the BT mesh system.
Particularly if the user is a BT subscriber, I'd have thought BT would be the sensible choice.If it doesn't work in their particular situation, I'd re-box it and send it back.
Yes that is what I thought but I had problems with mine and, perhaps worse, spent about a month going back and forth between BT mesh support and BT BB support.
At one time I was told you could not expect 8 devices to work on the network using it with BT Smarthub (including the three mesh points)!
My feeling is that the BT solution could be fine but BT's support at user (consumer) level needs working on. You could try it but any issues return it straight off.
I now have a Google Wi Fi - was not keen as ties you in to google eco system (you must have a google account). But support top notch and responses very quick to date. Pretty solid coverage - my house is not that challenging but one wall is a bit of a blocker.