Better bet may be a properly constructed investment club. Thousands exist and as long as the general invesment mandate does not prohibit spread betting you could be okay.
Which markets would you intend to follow?
7 year rule applies to Inheritance Tax not Income or Capital Gains.
Tax is not your biggest problem.
By accepting deposits on behalf of mates (investors) you are breaking very serious Banking laws. Remember spread betting is regulated by FSA and therefore to do what you propose you would also...