their webpage is o2.co.uk, you receive messages via email (firstname.lastname@example.org) and they can be up to 160 characterslong, I'm not sure if and how this is divided up though
Hope that's enough info.
i also noticed a few people had had problems trying to use the service so some instructions for it, they might help someone..
You need an o2 username (get one here)
o2 is the product of a genie.co.uk/BT Cellnet merger and so if you had a genie username then that should also work as an o2 username
Log in and you should find you have an email address set up automatically for you (email@example.com). You can check the inbox here.
If you just want to get texts from the livejournal service sent to your phone (ie: not spam or the o2 newsletter etc) you can enable a filter to do that here. You need to 'LiveJournal@livejournal.com' in the 'sender equals' box, enable that filter and save.
Then, on lj edit info page, you just select 'other' as the service provider and put your o2 email address as the number.
If you have problems following the links then the pages can be found under in 'my message pad' option when you've logged into the o2 home page. I've not had any problems with it so far but I know some people have had limited success setting it up. The site itself isn't totally reliable so be patient with it and it should work :)