Yes, I've seen a broken neutral on a ring frequently. A broken phase conductor also results in the same problem. I've also found "rings" with a break on all conductors, usually the result of a bodged DIY extension.
Rings most certainly can be protected by an RCD; indeed in some cases they would be required to have such protection (e.g. a house using TT grounding where there is a main RCD/GFI).
There has been a gradual move toward Britain adopting common standards with Europe over the last couple of decades (and in a few cases before that. e.g. adoption of the common brown-blue-green/yellow flex colors and metric size cables in 1970). Circuit breakers which were once rated 5, 15, 30A for example are now often 6, 16, 32A.
There are still stumbling blocks which the various national bodies disagree on, and the basic circuit arrangement seems to be one. The IEE remains firmly attached to the ring circuit with fused plugs, a system not used anywhere else in Europe except Ireland, and even they seem to be adopting Continental standards now.
