Hi,
I'm studying mathematics at university with an A-level backgroud of mathematics, economics and german. Although I got 3 As, it goes without saying that the more exposure to the advanced mathematical concepts you have before you enter university to study maths, the easier the transition will be. I notice that Warwick and Imperial among others ask students without further maths to look at and perhaps do(?) Advanced Extension or STEP papers as preparation for university mathematics. Saying that, university mathematics is conceptually different due to the fact that the courses have a strong emphasis on proofs which certainly get overlooked to a great extent in normal a-level mathematics but you get a bit more experience at dealing with proofs in further maths.
University mathematics without a further maths background will mean that the course will seem incredibly daunting at first. We spend the first semester going through most of the further maths stuff anyway but obviously you're asked to absorb the further maths syllabus at uni in a shorter time span relative to A-Level. It's not really the complexity of the work in the first semester that is difficult, more the amount of new things you need to familiarise yourself with. If you come to university without further maths but with good a-level grades, don't expect an easy ride. I think universities ask for higher grades in non-further maths applicants because they want people with a proven record of diligence and hard work in their school studies who will be able to handle and ultimately take in the barrage of new and unfamiliar theorems and conjectures relatively quickly. The first semester is very testing, as it was for me but eventually if you work assiduously and enjoy what you study, you should get to grips with the course eventually and then make inroads and excel in the actual post-further maths (university maths) stuff. Make sure to read through some of the further maths material in the holidays prior to the start of university if you're serious about wanting to study it. It does get quite fun and interesting after a while once you understand what you're doing.
Angad