Well you should not risk pursuing a degree which is not accredited unless the department concerned tells you can shift from your course. I think Motorsports & Mechanical are similar in a lot of ways, so you might as well have no problem shifting,but do ask them about it.
All the courses that I've applied to (see sig) are accredited by IMechE except for City's "Automotive & Motorsports Engineering". I've decided not to go for City because of this reason & some other reasons. But for most other unis that I've applied to,say Bath,allow changing from one course to another within the same department after 2 years (the first 2 years are identical for all the courses in Mechanical Engineering department in Bath).So you should contact your uni in this regard before taking any decision.
Now about the specialisation that you are talking about, obviously Automotive/Motorsports restricts your career paths but it does not eliminate them.In fact if you have got the interest,determination & passion for your chosen subject, then you are very unlikely to face employability problems.So I'd say go for it if you are up for it.Obviously,automotive/motorsports engineering is more attractive than just mechanical engineering (& add to it you have management modules too!). But just a warning: be ready to pay the price if you are not determined enough!