iirc, hurricane electric was offering IPv6 via tunnelling somehow.

https://tunnelbroker.net

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).

They do this for free because they're a tier 1, meaning everyone pays to connect to them and they benefit from having more things hosted on their network. It's free because your ISP pays for it.

And they can always tell your ISP if they don't like paying to connect to the tunnelling server they should just implement v6.