AFAIK the selection process for Britannia hasn't changed for at least 4 years so any historic information on the site would still be valid.
If you are invited you should receive a breakdown of what is involved with your letter. Here's what I remember:
Day 1: Numerical and Verbal Reasoning, Psychological profiling (nothing sinister, just the usual questions about otters), Group Exercise and finally a group chat (not assessed just information about the company).
All takes place at East Midlands and lasts for the best part of the day.
Good Luck