Very few people can launch into a "hard science" MSc such as human factors without first having a BSc/BEng - so in that regard I'd agree with badgerpuppy's assessment.
The only first degree in human factors that I know of is the
BSc in Ergonomics at Loughborough, however I think that's a full time course only and HF tends more usually to be a subject for second and third, i.e. more specialist, degrees. Your best bet might, in my opinion, be to try something like the Open University's BSc which allows you a lot of scope to pick the courses and create a mix that works for you. For example you could pick up courses in psychology, physiology, machine design, mechanism, statistical analysis, experimental method - effectively creating the degree you're after.
You could also try talking to
The Ergonomic Society who might know of a suitable course.
G