Piston engined aircraft and non-powered aircraft can fly VFR in No-fly zones. The following is from the CAA:
5.2 ATC and FIS Providers should not offer a service within a NFZ
unless the pilot is operating under VFR and the aircraft is a glider, balloon
or piston powered aircraft and not operating for the purpose of public
transport.1 This restriction does not apply to operational emergency
services flights (e.g. Air Ambulance).
ATC and FIS Providers may also provide a service to an aircraft in the
above categories within controlled airspace that is:
• operating under VFR within Class D airspace;
• operating under Special VFR in other controlled airspace provided
that it is the only aircraft in that airspace block