IO 540: Flying at 5000ft is all very well during the week, but at weekends, you'll find the sky close to UAS's full of Tutors between 4000ft and FL80 (roughly), and when Tutors fly at weekends, you often find it's without radar from the home airfield eg Boscombe and Benson, so no-ones going to warn them of your presence. Otherwise I agree; fly as high as you can without entering controlled airspace.