I'd suggest looking at some uni level maths textbooks/lecture notes/similar to get an idea of exactly what it entails - the maths done in a maths degree is very unlike that done in A-level, so even if you enjoy the kind of maths you're doing now in A-level Maths and/or FM, that might not necessarily be an indicator you will enjoy or do well in degree level maths.
The maths done in a maths degree is much more abstract (even the "applied" maths courses are often reasonably abstract), and often proof based and the proofs are a lot more involved than the very basic introductions given in A-level FM I gather. I'd suggest looking at some kind of introductory analysis textbook (e.g. Spivak's Calculus) or some introductory (abstract) algebra textbook. Some linear algebra texts might also be relevant if they are focused on the more formal/proof based approach rather than an introduction into the computational methods of matrix algebra.
The above isn't specifically to "boost your application" but to actually verify if the type of maths done in a maths degree is for you! If you find you don't like that more abstract type stuff but do enjoy the material you've been doing in A-level Maths, then you may find a course in engineering or physics more appealing, as they will include a lot of maths in the same vein as A-level (you'll essentially use A-level Maths as a starting point then go from there, and you'll use those methods throughout the course in all your modules).