I think Imperial is the only other one to use STEP?
No, an increasing number of other maths courses are using STEP, although this might change in a couple of years when the maths A levels become harder.
For example, see
www.ucl.ac.uk/prospective-students/undergraduate/degrees/mathematics-bsc
www.bath.ac.uk/study/ug/prospectus/subject/mathematical-sciences/entry-requirements/
For example, for Bath STEP is required for students who don't do double maths and is recommended for those who do. The carrot for the latter is lower grade requirements in the A levels. UCL also requires higher A level grades if STEP is not taken. Other RG universities including Bristol, Southampton etc include STEP as options in their offers.
BTW for Imperial you can also take Oxford's aptitude test, the MAT, which is taken in the Autumn of year 13. The MAT however is slightly more poorly correlated with future outcomes than the STEP i.e. STEP more fairly picks out the best mathematicians.
Obviously, not getting STEP when you're a teenager doesn't stop you from becoming a mathematician, let alone studying Maths.
This is true. Nonetheless, you are substantially more likely to become a mathematician if you go to one of the "top" universities which require STEP. Moreover STEP is far closer to university maths than A level maths, so preparing for it bridges the huge gap between A levels and university.
Warwick promise a simple offer to everyone of AAA plus STEP1.
This is not true. They promise such an offer to everyone who has the required predicted grades for A2.
are there resources or tutors? however we are a low income family and we will struggle to afford tutors
There are lots of resources on the STEP website. The main thing is to work through past papers and their solutions. The mark scheme is available and should not require teachers to mark it. I would recommend against using a tutor. Imo the only people really qualified to tutor for STEP are maths academics, in any case, who don't tutor. But more importantly STEP is not an exam for which tutoring helps. It is an aptitude test in which the questions are deliberately designed to be "original".