QXC
I included Gloucester in my QXC - quite a nice airport to find and land at. They also do much reduced landing fees if you tell them you are a student. Paired with Bristol as a first look gives you a 154 mile route.
As a suggestion - I purchased the CAA charts on memory-map. Allows you to draw your route electronically. This allows you to modify your route easily and be told the total length at any time without lots of rubbing out & redrawing.
Could also be worth considering airports with multiple runways or at least runways in similar directions. Otherwise if its a windy day you might have to cancel because at least 1 runway will have a strong crosswind