The semi circular rule can hardly ever be used in the average spam-can in UK. If you want to fly at FL085, it will take you 20 minutes to climb to that altitude at cruise climb speeds, and a further 20 minutes to descend. So for 80 miles you will be either climbing or descending... For most of UK you can hardly find 80 miles of clear airspace without bumping into controlled airspace. At lower levels your altitude will depend more on terrain and cloud avoidance.