A lot of things involving electricity in places with type-G plugs seem more dangerous if you don't know/assume that almost everything is grounded. Took me a while to realize that hairdrier he mentioned in the blogpost likely didn't have a ground connection in the plug.
In the case of a toaster with a grounded frame, the most likely route for a short is straight to earth which will trip the RCD on that set of plugs.
Toasters (usually?) don't have RCD (what they call GFCI in the UK) in the US. Maybe your circuit will have a GFCI, but it depends where you put your toaster. Do all circuits have RCD in the UK?
Most buildings that were built or have had substantial renovation work done will have RCDs installed, yes.
While you can have a separate RCD for each circuit, and the newest installations do, it's more common to have two RCDs in the panel/fuse box/consumer unit, with each circuit on one or the other. This arrangement is known as a 'split load consumer unit'. The intention of having two instead of one is that plugging in faulty electronics at night not plunge the entire house into darkness. These RCDs usually have a trip current of 30mA, making them less sensitive than American GFCIs which trip around 5mA.
In the case of a toaster with a grounded frame, the most likely route for a short is straight to earth which will trip the RCD on that set of plugs.