Now let's not confuse matters. Watt invented horsepower. He was named after the unit for power! And I'm not sure he was a knight, either. I blame the French for inventing SI. Incidentally, I do that joke for the kids at school quite a bit. We also do the 'what is the unit of power' joke on a fairly frequent basis.
Don't the americans typically rate theirs Programme and the brits SPL or something? Last time I bought a speaker the retailer had kindly supplied a chart comparing 3 different ratings, all conforming to some standard.
Whilst everything you've written is true, all other things being equal (ie resistance(s) being the same in the circuit) a doubling of p.d. ought to give 4x the power, not twice. Do we Brits crank up all the resistances of things by root 2 to compensate?
{ breaks out scientific calculator} .... This reminds me of something my Professor would have asked in Electronics school.
It does look like there's some sort of confusion about the fact that standard British voltage is twice standard American voltage at the outlet. However, Watts are Watts so that should be a set value. Now, maybe it has to do with effective current handling ability as 30 W at 115 V would require double the current of 30 W at 230 V. That could explain it.
As far as I can tell, the British rate watts with a completely distortion free sound. Americans, not so much.
RMS vs. peak. If you take Ohms law: E = SQRT(P * R), punch in 30 for P (watts peak) and 8 for R (resistance) you get 14.49 volts peak. Now convert that to RMS and you get 10.95 V RMS. Now use ohms law to calculate the power. P = E^2/R and your get P = (10.95^2)/8 = 14.98 watts RMS.