Overrated: Any London uni not named Royal Holloway (I know it's technically not in London but its part of the group), Imperial College London, or Royal Veterinary College. Specific focus on LSE, UCL, King's. Just go to Durham instead.
It's usually not worth the astronomical cost of living, and often suffer from very poor satisfaction rates. I mean god when you're LSE and ECONOMICS has a 79% "overall satisfied" you know something is wrong. Also they tend to mire around in the bottom end of the annual THE Student Experience Survey. Only the three listed above are at or above the national average. Still expensive to go to, but their course quality is of a much higher standard.
Underrated: Bit tougher. Generally I'd bring up QUB since it is for some reason considered the worst RG uni by many, and it's location turns people off. It has a beautiful campus and puts alot of effort into its facilities. Belfast is also the cheapest city to live in the UK.
Other than that, Lincoln is a massive success story. They've raced up around 40 places in the last 8 years and have broken into the nation top 50 (still going up). The staff powered this uni forward from being an ex-poly to a rather decent university.
Plymouth University is another that I consider a good ex-poly. Carved themselves out a big niche doing marine stuff, and they are good in healthcare and medicine related stuff too I've heard.