G5 is reputation. This is not the same as REA ratings, or number of 1, 2.1 ratios, etc.
The best way to see reputation is at the world league tables, by THES.
This table rates the view of the top 1300+ scholars in the world, and is weighted as 50% of the overall rankings thus a far better representative of reputation. In this case, it Oxbridge 4/5th. LSE is 1tth, Imperial 14th, and UCL just below hence the g5 unis.
However, it shud also be noted that reputation is not the same as 'the best.' As you have allready mentioned warwick has great REA ratings, etc.
No-one can give a definitive list of the "top 5" universities as such, as it's such a subjective thing... Surely the most important thing is what matters most to the individual, if the course suits them etc - although I agree it's a nice bonus if the uni also has a good reputation from the point of view of later employability, etc.
Personally though, these are my 5 favourites for English/English&History:
Apparently Sussex & Warwick are excellent, but I don't want to go to either! Durham looks lovely too.
In terms of financial power then it's: Oxbridge, UCL, Edinburgh and the new combined Manchester Uni.
EDIT: I've treated ICL separately because it is a specialist institution.
I thought warwick was reknowned for its 'financial power' and is one of th uk's richest univeristies. doesnt it generate huge amounts of income for itself wihtout having to rely on the government for funding? or have i imagined that...
How do you manage with all the debt? Are you rich? Have you become institutionalised? Who funds you?!