A belated reply to @Decorhate. Very much a layman's view so happy for others to contribute. Psion (sp) will know a lot more than me about mathematical economics courses and alternatives.
Like many subject degrees, there are huge differences in course content between one University and other. Which does not mean better or worse. Just different.
I think the trick is for him to think one step beyond, and consider what he might want to do with his degree.
For DS this was reasonably simple. He was fascinated by economics and potentially wanted to be involved in economic forecasting. Lots at LSE simply wanted to work for Goldman Sachs and earn lots of money. Others wanted a good understanding of economics, along with other management type skills like being able to read a balance sheet or understand the workings of a marketing campaign. Then you get the interfaces between economics and history, economics and geography, economics and politics, or economics and psychology etc.
Back to supply and demand. There is an awful lot of data underpinning the more technical understanding of economics. There are only so many A level student with the aptitude to gain a good grade in FM, and who want to study economics. Economic forecasting is important for Governments, banks, hedge funds, consultancies etc, and so well qualified quants are in high demand.
Generally four Universities are recognised as having the most mathematical courses: Cambridge, LSE, UCL and Warwick. I assume in part because there are only so many strong mathematicians available to be recruited. DS' experience was that the maths went at a cracking pace. He says they covered half a FM paper in a single lecture. He struggled a little in his second term as the field is quite narrow and concepts tended to be iterations on a theme. His tutor was kind enough to give him some useful study tips, mainly preparation in advance of a lecture, practice afterwards and revise regularly.
LSE has lots of options. DS graduated, as ChasingSquirrels DS wants to, in Econometrics and Mathematical Economics, which is as quantitative as you can get. He took more maths courses than his friend studying engineering at Imperial, and indeed reckons you can get away with only taking two economics courses out of the 12 needed for a degree. A smallish tip is that you can get onto Econometrics from Maths courses (Maths with Economics etc) which can be slightly less competitive at UCAS stage, but either way you need to make sure you have taken the right courses in your first and second years and get the right grades. Most students don't want that much maths, but even still LSE, and I assume the others, have some tricky compulsory maths courses in first and second years.
DH studied PE at Oxford, which he claims is real economics. (I also went to LSE so take DS's side.) Back to depending on what you want to do. If you are recruiting literate policy makers who understand the language of economics, then PPE is a better bet. Bath has some great courses with economics and scope to study accountancy and finance with a year in industry and a good reputation with employers. York is similar. And lots: Bristol, Exeter, Durham, St Andrews, Edinburgh, Nottingham, Southampton, and, and and, have well regarded broader less quantitative courses where graduates go on to be recruited by good employers.
And if you really want to go down the quant route, you probably need to take a Masters, so there is always scope to switch to somewhere with a stronger research focus.
HTH.