I don't think there is a list as such.
The only way to be sure is to create a UK Roku account online (you must physically be in the UK, i.e. UK IP address, in addition to UK postal address and UK CC number, otherwise it will set up a US account).
Once the account is set up and you are logged in, go herehttp://streamfree.tv/apps/roku-private-channels/
select a channel and see if it installs, then repeat for each desired channel. I'm fairly sure that this method will work without actually having linked a Roku to your account, but am not 100% sure.
Otherwise, you'll need to identify UK users here in the forum and ask them to try to install the channels you would like to see.