You do not pay any supplements in the United Kingdom.
This is realy relaxing.
Only in night trains, you have to pay for your beds. And this not cheap ... but okay if you want to sleep one night very comfortable ... I remember London - Edinburgh was about 40 EUR for a two-bed-compartment per person.
But you do not have to do it by nighttrain ... the connections from London in direction of Scotland are very fast and easy to do within a day.
(compared to InterRail):
- you can select, how many days you want to travel (very flexible)
1,2,... days within a month
- you can not travel Ireland
in my eyes, Britrail is a little bit more expensive than interrail, but more flexible.