If one of the teams relegated is the host fort he following year then they take the lowest 7th place team. If both are in the bottom then it's the two 7th place teams. I remember reading it somewhere. So GB need to best Belarus and then hope Latvia don't do worse than 6th. Then they have a chance.
This has changed. From the tournament info:
Relegation format
The overall bottom ranked two teams will be relegated to the 2021 IIHF Ice Hockey World Championship Division I Group A with specific rules in place for next year's host nations.
If Belarus and Latvia need to be protected from relegation from the 2020 IIHF Ice Hockey World Championship, only one team from the 2020 IIHF IIHF Ice Hockey World Championship will be relegated and only the winner from 2020 IIHF IIHF Ice Hockey World Championship Division I Group A will be promoted.