Technically:
according to research conducted by (I think) UCAS which I don't have a source for unluckily, SL maths at IB corresponds to about 2/3 of a full A2 level maths course with 4 core and 2 applied modules.
That said however, that is purely content-wise. A level exams are split into 6 separate examinations over 2 years, so you take the syllabus in chunks. Especially the applied parts such as stats, mechanics and decision are completely separate from the core. In SL maths at IB you have to know the entire syllabus for 2 examinations, one with and one without a calculator. You will have to be confident on the entirety of the syllabus to get a good mark, whereas for A levels you can revise in "chunks", as the core is separate from the applied modules and the core itself is split into C1/C2 in year one and C3/C4 in year 2.
Having only done AS maths in year 11, I would say that at least the first half of your exams in A2 maths is extremely mechanical, i.e. the questions you get in the exams follow a recurring trend, which is pretty easy to learn and prepare for. This might be a bit different at A2 level (would be nice for some A level student to clarify), but half of your A level grade will be determined by exams which are very easily revised for, as they are essentially the same questions every year with different values. In IB SL maths you won't have that luxury, as both your exams necessitate knowledge of the entire syllabus and the ability to apply your knowledge onto situations you haven't seen before, though not quite as much as in HL maths.
Bottom line: if you want a comparable difficulty to A2 level maths, go for the IB SL course. It's general opinion that maths HL is the hardest IB subject after further maths SL, and you probably shouldn't even be considering HL maths if you think it might be too hard. English might be the safer bet, if you're not really confident in either.