How are British trad grades determined on this site?

On the route guide there is an option to select multiple grading systems. I have a friend in the UK whom I talk to about climbing often and so I sometimes use the British trad grades from the route guides here.

The British trad grading system seems more complicated than YDS, so I'm wondering how the British grading is calculated? Is it a straight chart conversion, or is there input from people familar with that system and the routes?

For those familiar with the British system, and who have used that feature on this site, how accurate would you say the ratings are (discounting the usual issue with soft vs hard ratings that is often talked about)?