I studied Further Maths at A-Level. It's not prohibitively difficult, I found it to require about the same total amount of work as my Chemistry A-Level.
But you do have to be very mathematically inclined. It's designed to be a genuine challenge for the kind of student who cruises through to a top grade in the normal Maths A-Level without breaking a sweat. You know, how there's always those one or two nerds in the class who just seem to "get" Maths without trying, and are on another level compared to everyone else... Those sorts of people have to actually study properly for Further Maths as they would for any other A-Level, because they can't just get by on natural ability anymore.
If you can very comfortably consider yourself to be one of the top 10 mathematicians in your year, I'd say go for it.