According to PPL Air Law, the Quadrantal Rule applies upto FL245 and then the Semi-Circular Rule applies above FL245.
So does this mean the highest available FL for a heading of 310 degrees using Quadrantal, would therefore be FL225?
You're a bit out of date. If you look at CAP393, Rule 34 (11 Jan 2010 revision) shows the division now as FL195. So FL185 is highest usable as a cruising level for 310 degrees.
All airspace above FL195 is class C, hence you do what ATC clears you to do.