The CAA has published guidance on the use (or not) of GPS in GA aircraft in SafetySense Leaflet no. 25 "Use of GPS".
Probably the biggest risk of infringement, when using GPS, is when flying way-point to way-point using GOTO where the track passes close to CAS and there is no way-point near the CAS. The GPS may show that you are less than 5 degrees off-track - but if the way-point you are navigating towards is 30 miles ahead you may be a couple of miles or more away from where you thought - plus the 2 km GPS error claimed in the leaflet. The solution is to put a control way-point at the nearest point to the CAS that you want to be to ensure that you don't infringe. A few degrees off-track 1 mile from a way-point is negligible, 30 miles or more away is problematical.