I was doing this back when I had BT in London, my internet speed back then was pretty abysmal and it would randomly drop routes- ironically this tunnel actually improved my internet (because of the routes not being dropped, not the speed).
And they can always tell your ISP if they don't like paying to connect to the tunnelling server they should just implement v6.
It wasn't always like this, but it was for me the last time I tried.
I've got a 8Gbit symmetric fiber connection (with no IPv6 at all, yay) and regularly make use of all of it, so I'm well connected. Router hardware is more than capable of the encapsulation.
With the HE tunnel up, I was often getting tens of megabits per second for any endpoints that supported IPv6.
Works as a "this endpoint is _only_ available on IPv6" so I can connect to something I couldn't otherwise, but day-to-day the experience is vastly better over IPv4 for me.