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.
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.
As far as fluid contamination goes although the rubber diaphragm is probably squeezed pretty tightly between the body of the switch and the plastic part forming a seal, maybe silicone can squeeze through that, and maybe modern switches don't compress the diaphragm as tightly anyway, and maybe the contact material is just poorer. But these (a very old switch much before my time 21 years ago, and quite possibly original) look pretty flimsy anyway.