Lancaster is really, really underated on here.
Lancaster, mainly underrated because no one knows where it is or thinks its hard to get to, which it isn't.
Queen Mary. Its a lot stronger than many think and they performed exceptionally well in the 2008 RAE assessment. And graduate starting salaries and employment prospects are high too.
I think the question about which universities are underrated needs to be answered in a given context really.
I'd say in the general public (i.e. people which don't really have the slightest clue about universities), places like Manchester, Sheffield and Newcastle (in other words, just solid, research intensive universities) will be overlooked as just: "universities that aren't Oxbridge". When in actual fact, they are much stronger academically (in a range of ways) than places like, say, Portsmouth or Plymouth which would probably be bundled into the same "the rest after Oxbridge" group by many.
"Down south", I'd say most of the universities "up north" are underrated (which is to be expected too). For example, when I got rejected from Durham people I know were sort of like "wtf, you got offers from Exeter and Southampton but you got rejected from Durham" signifying a clear regional bias and lack of knowledge.
On TSR, I would say the Scottish universities (Edinburgh and maybe St Andrews aside) are fairly underrated. People seem to ignore the fact that Glasgow and, perhaps to a lesser extent, Dundee and Aberdeen are ancient, academic powerhouses.
Overall (and going back to my first point), I'd just say that solid, traditional universities often get overlooked because they don't carry the Oxbridge/London brand.
Queens University Belfast!
I don't see why Nottingham Trent is underrated.
It is very good for "new degrees", but e.g. English and History is not so well.
Leicester is very underated. If it wasn't for this unviersity, DNA wouldn't exsist!!!!!!!!!!!!!!!!! However, it doesn't seem to have any 'wow factor as other unis which are of equal acadmic level like Manchester, Leeds and Nottingham.