Hi,
I've been self teaching a levels, including maths with mechanics, for the past 2 years and am just finishing A2s now. In terms of mechanics, it is quite heavily physics based, but is only covering a narrow range of physics (forces/momentum/moments), though it covers it to significantly more depth than a physics a level does (which I'm also studying). Though I found the mechanics exams to be much more about using algebra & core maths skills to solve unknowns, rather than having to have a great deal of understanding on the physics side (fairly easy to remember the rules & formula).
In terms of self studying, it's always gonna be harder than it would be in school, the main difficulty is motivating yourself to put the hours in, then there's the obvious that you don't have any teacher to turn to if you get stuck. I'd recommend doing your a levels through a distance learning site (I did it with NEC), as they give you what is essentially a text book, with all the resources/questions you're required to study, as well as tutor support, so have someone to contact by email if you get stuck, and there's also a few marked assignments for the year (though nowhere near the same amount of marked work you get in school).