Generally, the plans specify the outer dimensions. IOW you can cut the given tube square and then mitre the ends. It's easier to make a mitred cut from the outset, though, and because many tubes are paired, you can reuse the mitre on the remaining tube, too.
However, mate, have you got the right plans? Tube C? Sizes in inches? You must be looking at the Uncle Ron's book drawings, and you need the Chris Gibbs ones if you want to follow the SSC MX-5 guide. The Gibbs (Haynes Roadster) chassis is 4" longer, 2" wider and 1" higher than the Uncle Ron/Locost one.
http://www.amazon.co.uk/Build-Your-O.../dp/1844253910