Note: this is entirely theoretical. I have no (current) plans to actually do this.
Would I violate any state/national building/electrical codes if I installed BS 1363 (UK plug/socket) sockets on a 240v circuit in an ordinary residence? In addition to the existing 120V circuit with NEMA sockets.
For the sake of the question, assume that the goal is to wire the 240V sockets as closely to the way they'd be wired in a UK house as possible. (mostly with respect to safety features)
Failing that, what features of a standard BS 1363 outlet can I implement which differ from a permanently-attached NEMA-6 to BS 1363 adapter?
And most importantly;
What are the safety implications of installing/using a setup like this?
Inspired by this question about running 240v circuits and looking into the NEMA plug types available in the US.
Best Answer
Mutually Incompatible Certification Schemes
The primary problem with what you describe (installing a foreign-standards receptacle into a North American electrical system) is while foreign receptacles of reputable make (vs. some Cheese-pipeline special) are going to have a third-party certification, that certification is going to stem from a different certification scheme than the OSHA NRTL (Nationally Recognized Testing Laboratory) scheme we have in the US. For your BS1363 receptacle example, the third-party certifications for these receptacles are going to come from an IECEE (IEC System for Mutual Recognition of Test Certificates for Electrical Equipment) Certified Body (the European analogue to a US NRTL), as seen in this example for a Legrand Arteor BS1363 receptacle. Most US Authorities Having Jurisdiction (AHJs, aka local building inspectors) will not recognize such a certification without prior research and written agreement (at minimum, some may simply flatly refuse to recognize anything that does not fall within the OSHA NRTL scheme), just as Building Standards in the UK isn't going to wave through say an Intertek ETL listing as "approved" without looking into things further.
Parts do exist for this...
However, amazingly enough, there is such a thing as a switched BS1363 receptacle on a North American yoke; I contacted Leviton tech support, and while you'd have to contact them via email or webform I presume to obtain the certificates, the tech support contact did indicate that the part linked has a third-party certification through the IECEE scheme, so it wouldn't be a completely lost cause for a written AHJ variance.
...but likely aren't needed, either
However, most appliances intended for use with BS1363 plugs either come with a stripped/tinned cord end so that the appropriate plug can be fitted, or use an IEC appliance coupler and mating detachable power cord. In these cases, there's no reason to use a foreign receptacle type when the appliance can simply be fitted with a NEMA 6-15P instead; even if the original cord and plug was hard-wired on, chopping the plug off, stripping back the cord, and field-fitting a NEMA 6-15P would be more reasonable than trying to go through the hoops of getting an AHJ to sign off on having foreign receptacles permanently wired into one's house.
As to that L-L vs L-N thing...
There are two other differences between US 240V and IEC 230-240V as well; while mains frequency (60Hz vs 50Hz) is one of them, it's not feasible to convert between the two, and fewer and fewer appliances are frequency-sensitive in this day and age due to the replacement of electromechanical drum timers and capacitor-start AC induction motors with electronic controls and inverter motor drives. In any case, if the appliance says "50-60Hz" or "47-63Hz" on its nameplate, than it has been tested to run on both mains frequencies and will not have an issue with the change.
The other difference is that US 240V power is Line to Line (two hots and a ground), while IEC-style 230-240V is Line to Neutral (hot, neutral, ground). While a NEMA-style dry-type distribution transformer can be wired to convert between the two, the use of 240V-to-ground power in the US requires that 240V straight rated breakers be used for overcurrent protection (vs the 120/240V slash rating found on standard North American circuit breakers). The good news is that 240V straight rated breakers are made, although their availability depends on which make/model of panel you're using (offhand, Square-D Homeline is the only panel line that does not have straight rated 240V breakers available for it); however, special functions such as GFCI and AFCI are not available in 240V straight ratings, which puts a hard limit on where this adaptation can be done.