> the "ring main" wiring approach, which is mandated by British Standard number...
I have a 1938 UK wiring manual, and there is NO hint of Ring Main inside the premises. (They do show one connecting several sub-stations.)
> electrical regulations allow for 3-conductor 240V, PVC-insulated power cables to be buried just beneath the wall surface.
And worse. Like rubber insulated cable. Remember: 1938. No modern Neoprene or other synthetic polymers, just tree-sap cooked with a little Sulphur.
> the US practice -presumably mandated in the NEC- of grounded romex or grounded conduit
Never been mandated by NEC. By the time knob-and-tube (open) wiring was going out of style, various NMC (non-metallic cable) systems were available.
NEC does require conduit where the wiring may be damaged. But aside from special cases like gasoline stations and mines, it does not say what risk of "damage" requires conduit. That is left to the Local Authority. (We must note that the NEC has no authority; it is just a convenient foundation for Local Authority to adopt and adapt to local conditions.)
In my school, ALL wiring is conduit. Won't even use 3 feet of metal-flex, just the 18" that the NEC suggests for connecting to motors and lamps that might move. My observation is that most commercial work around here is also in conduit. Residential work is NEVER in conduit around here, it is all NMC: two insulated wires plus a ground enclosed in a PVC jacket.
I have heard that in certain cities, full metal conduit is required for all electricity. This may be extreme fire prevention in densely-packed residential areas, rather than job-protection for the Electrician's Union as some people think.
> you mean PVC tubing for domestic jobs?
There is a grey plastic tubing widely used for outdoor runs where plain NMC is not a good idea. When used with its glued couplings and gasketed boxes, it is mighty close to water-tight, more so than simple EMT (light metal tubing with pinch-screw or compression joints). It isn't as solid as true screwed conduit, but it is a heck of a lot easier to work with.
I should think that an inside run in low-density Florida would be plain NMC fished through walls or in plastic WireMold. No metallic shield, but the cable can be had Twisted or untwisted. To comply with the White=GroundED rule, a 240V-only run probably should be wired with 3G cable: White-Black-Red-Green, with the White ignored if not needed. That's wasteful, but using the White in 2G white-Black-Green cable as a Hot is bad form. (Yes, you can permanently un-White it and use it as Hot.) And 3G cable is usually Twisted, while 2G cable is often "twinlead" with no twist except whatever you get by pulling it out the inside of the reel. The 70-foot run of 3G cable that I ran to my stove was not expensive compared to my labor. And that cable was rated 45 Amps peak (only for stove duty, I think 30A for general duty, which is more than Bob needs.)
Stove-cable is probably good. Not that this 11-amp load needs 30 Amp cable, but the wire must be fused and as you know it is hard to find double-pole breakers smaller than 20A or 30A. Also you want a FAT ground wire. Note that large-gauge cable often has Ground one gauge smaller than the hot wires, so you may need "overkill" for a fat ground. Upstairs, you might want a 10A or 15A slo-blo fuse to protect the transformer, though unless you run A/C off this tranny I doubt you would ever pull 2,500 Watts of load without noticing HEAT. (That's like two electric room-heaters.)
I used to lose a modem and an answering machine every summer, from lightning. The house once had overhead phone and power lines. When they split the lot, they put in new underground services to each sublot, so I had underground power and six underground phone lines. BUT they never cut-over the phone service and it still came in the overhead line. So the overhead line caught lightning, which headed toward the grounded electric service. But the phone protector was grounded to the water pipes, and they had installed a plastic water line up to the house. So I had 6 inches of dry dirt as phone "ground". The "best" path for lightning was through the modem, and when that zapped the next-best was the answering machine.
Finally it ruined the 1930s heavy-porcelain carbon-block protector so bad that I could not hold the line. Got a guy out, and told him that this overhead line was stupid when I had 6 underground lines. He went beyond the call of duty and cut my line over to underground, and took the overhead line away. That modem lasted five summers, and was retired only because I got DSL.
As Bob says, the loop-area between the audio (phone) and power may matter. I had a 20'x100' loop, reduced to a 3'x100' loop plus dirt shielding, and it made a difference.
> Dirty in the sense it touches neutral before it makes it out to the earth
That's Required. All Whites and all Grounds connect at the Main Entrance(*). Not upstairs in the sub-panel! And you can't go from studio, to dirt-rod, to Main Service: it must run studio to Main Service to Dirt.
Dirt in Florida is useless. Damp enuff but pure sand. Bond to it to reduce outdoor shocks, but you won't get enough ground to swallow lightning. A large metal piping system would be good, but has been rare for decades. Even my gas comes in via plastic. Unless you can bury a couple of Cadillacs, salt the earth, and wet it in dry months, you are just sitting on damp powdered glass.
(*) Exception: with a true transformer you have a Separately Derived Source. You can bond all your studio Whites and Grounds in that distribution box, and run one Ground wire back to the White+Ground bus in the Main Entrance.
But DANGIT... go to the library, they have the NEC in the Reference section. Or go buy a copy.... I get a new one every decade or so, even though my house is still catching-up with the 1950s. And get an Annotated edition. The actual NEC assumes a lot of jargon, you need interpretation and context. I like McPartland NEC Handbook, McGraw-Hill. The 2005 edition is ISBN: 0071443401 Amazon.com has it at $60, or $122 with NFPA's annotated NEC ISBN: 0877656231 (a good pair); both are available for less via used-book sites, especially if you take a 2002 edition (basics change slowly, though for swimming-pools and hot-tubs you MUST be up-to-date).