For Maths courses, virtually every top university will either require it (as Durham did which I’m off to in a few weeks) or give a lower offer to people who do it. I’m not sure about the specifics but I know Oxbridge, Imperial and Durham definitely require it whereas I believe Warwick, Bath and I think UCL (?) give reduced offers to people who do Further Maths but still accept people who don’t do it. Warwick’s standard offer for no Further Maths and no admission tests would be 3 A*s whereas their standard offer for Further Maths and a good grade in an admission test (TMUA, STEP etc) would be A*AA.
For Economics I think less universities require it, but particularly top-level universities would prefer it if you did do Further Maths. They might give you a reduced offer or just generally feel more inclined to make you an offer but I can’t speak too much about this as I didn’t apply for Economics. I heard some universities, particularly LSE, are a bit funny about Further Maths being one of 3 A-Levels, so I would maybe recommend doing Further Maths as a 4th subject if you were thinking about LSE. I regret not doing 4 A-Levels myself as I felt like what I learnt in my 2 years felt really narrow and the workload was, for me personally, more than manageable.
Further Maths is obviously hard but it’s definitely not as hard as what people make it out to be. A lot of this might hinge on the optional modules that you do, for example if someone was doing Physics they would naturally find any Further Mechanics easier, and I was at a bit of an advantage in Decision because I did Computer Science. It is one of those subjects that is all about practice, it’s concept-heavy but you will get out of it exactly what you put in. Grade boundaries can also be very volatile so please try not to compare how you do to them but just strive for as close to full-marks as you can. Good luck!