Note that late UK cars seem to have a subdivision of the green circuit with its own in-line 35 amp fuse supplied by the white/brown (ignition relay) circuit feeding things like heated rear window, turn signals, heater fan and tach, which leaves the original green circuit fuse (2nd one up in the four-way fuse block) feeding things like reverse lights, stop lights, washers, wipers, and circuits associated with the seat belt warning lamp and time delay buzzer.
Brake Light Relay
Relays with diode protection are available off the shelf, see the 30 Amp Relay With Diode 72714 on this UK site, or the 50 Amp Sealed Automotive Relay With Diode R-50ASD on this North American site. These both have a single diode across the winding which protects against reverse voltage spikes, but there are other types around with a second diode in series with a winding terminal which protects against incorrect connection as well. The types linked above will protect whatever operates the relay, but you must be sure to connect the relay winding the correct way round or you will blow a fuse and/or the diode and/or the switch that is operating it. The series diode in the second type protects against reverse connection as well, if connected the wrong way round the relay simply won't operate. When installing the first type bridge the terminals on the brake light switch or the spade connections on the wiring before pressing the brake pedal (ignition on of course). If the lights operate without blowing the fuse then reconnecting the wiring to the switch and operating the pedal should be fine. If the fuse blows, the relay is wired the wrong way round, and you may have blown the internal diode. You can protect against that by wiring a 12v bulb in series with the relay winding when testing. If the bulb glows at full brightness the wiring is reversed, but won't blow the fuse or damage the relay. If the relay clicks and the brake lights come on, it is connected correctly.
If you mount the relay close to the switch you pick up the ground from its mounting, and if you use a
connector to pick up the green circuit everything can easily be restored to normal very easily if required. But if you connect terminal 30 to the purple circuit at the fusebox instead of the green
or via an in-line fuse to the brown,
your brake lights will probably be slightly brighter and you will suffer less from the indicators slowing down when the brakes are applied.
Any automotive accessories relay should be suitable. You may be able to get one that includes the diode but this is not usual and you will have to add one. In both cases it is essential to get the diode connected the correct way round
or you will blow the green circuit fuse, possible the diode, and almost certainly your new brake light switch. As shown it is correct for negative ground systems, the older positive ground systems will need to have it wired the other way round. The diode must oppose normal current flow in the circuit as the only time it conducts is in the presence of back EMF from the relay just as the brake light switch contacts open, and that current is in the reverse direction from normal. The diode 'shorts out' the back emf and prevents the brake light switch contacts from sparking and welding just as they open.