How do I change the timezone to UTC?

sudo dpkg-reconfigure tzdata

then go to "Etc" or "None of the above", there to UTC, enter, you're done. It actually changes files /etc/timezone and /etc/localtime.

Unable to find UTC or GMT in the menu, I ended up running tzselect from the command prompt. I selected 11, enter posix standard time, then entered UTC-0. This appears to have done it.

thanks for all of the help.

This is an ugly hack, but it appears that Iceland uses year-round UTC. So setting the time zone to Atlantic/Reykjavik should work (at least until Iceland decides to change its default time zone).

(London doesn't use year-round UTC, so that's not going to work correctly.)