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.
Our free tunnel broker service enables you to reach the IPv6 Internet by tunneling over existing IPv4 connections from your IPv6 enabled host or router to one of our IPv6 routers.
https://tunnelbroker.net/