Since never, unless of course you want to enter controlled airspace.
Ah, but since this thread is addressed to 'rules knowItAlls', have you read the definition of controlled airspace in Schedule 1 to the The Air Navigation Order 2016? It says: "
"Controlled airspace” means airspace which has been notified as Class A airspace, Class B airspace, Class C airspace, Class D airspace, Class E airspace, Class F airspace or Class G airspace;"
After checking that reference you may want to read ORS4 No. 1238 - General Exemption E 4536 which is valid to 30th September.