I fly a lot over a local nature reserve which is in a FL 85 / 25 zone about 10km from a UK international airport. I see plenty of commercial airliners flying in towards the airport, according to my android app they are at around 3000' when they are nearby. However the nature reserve attracts quite a lot of small aircraft such as helicopter (police/rescue/other) and light fixed wing. When I fly I try to stick to 100M (i.e. below 400') and listen and watch all the time for other aircraft, but should these small aircraft be flying above a minimum height in any case ? Which CAA legal instrument defines this? Thanks.