Mastodawn

Deutsche Telekom:
#roaming works with #ipv6 in #uk with ee, o2 and vodafone
MTU is lower (1440) than at home(1500).

(The #IPv6only mode (standard at home) is still not supported)

Vodafone (de):

roaming works with #IPv6 in UK with Vodafone (uk)
MTU (1464) is lower than at home (1500)

o2 (de):

roaming works with #ipv6 in UK with o2 (uk)

MTU is 1500

Dualstack is provided at home and during roaming.
#IPv6only (really only, no NAT64, no DNS64) is also working at home as well during roaming in UK.

1&1 (de)

roaming works with #IPv6 in UK with EE.

MTU is 1500

Dualstack and IPv6only (with NAT64, but without DNS64) is supported. (same as home)

#iOS still doesn't work out of the box.

But the right profile helps.
And after changing the hotspot APN the hotspot works too.