Changing frequency is not very practical at anything other than very low powers. Conversion to DC and then inverting, or in old school method, for lots of power, motor-generators are the only sensible ways.

What sort of loads are they intending to connect ?  most office electronics does not care about 50Hz or 60Hz, and probably does not care about 110V very much either- things like laptops and monitors and printers etc are universal mains these days. Anything big like a heater would be better to buy one over here.

A US voltage UPS may be a way forward if there is some bespoke kit that really must, but I'd be pushing back.

  UK 110V building site supplies are center earthed while US 100V has one side as neutral so the possible shock to ground is doubled. I think some RCD would be needed - but then the Americans have GFCI that are more or less the same thing.

From an installation perspective unless the building is owned outright, you probably want anything you do to be more like an appliance or a rack and some extensions rather than full blown fixed wiring.

Things like US airbases and embassies in UK and mainland Europe have US style supplies, but  they are not properly under  local law. Certainly when I was last on a US airbase, and that was in the last century, they had their own 60Hz generators for the site und once inside the armed guards all installation was to US code.

I suspect a normal UK  office does not enjoy that freedom- or budget.

Mike

Will the installation after the step down transformer be regulated under BS:7671?

In BS 7671 terms, yes - it covers electrical installations generally unless displaced by a more specific standard. I suppose there is a question of whether you need to adhere to BS 7671 - or whether another standard would be more appropriate (e.g. the US's NFPA 70). That would have to be answered in conjunction with the likes of the Safety duty holder and the likes of insurance providers.

In principle there's nothing in BS 7671 stopping you using 110V sockets - mostly the same requirements as any other LV socket circuit would apply - although some of the details (e.g. table 55.1) are likely to pose some challenges - but they might be circumventable with a bit of imagination, I can certainly see an argument that danger might arise from plugging a 110V appliance into a 230V circuit for example.

  - Andy.

Something like sticking a row of yellow 110V commando sockets on the wall.  Then making up a load of extension leads with commando plugs on one end and US NEMA sockets on the other?  Because extension leads aren't covered by BS7671.

Because extension leads aren't covered by BS7671.

I'm not sure that's the case here - BS 7671 hasn't been limited to fixed installations for a very long time now., but applies generally - unless superseded by another standard. The likes of 13A extension leads covered by a more specific standard (BS 1363) - but I doubt there's a corresponding standard for BS EN 60309 - NEMA leads.

Also if adopting US style socket outlets, consider US style methods - e.g. 6mA rather than 30mA RCD (GFCI) protection and possibly even AFDD (AFCIs) built into the 'receptacles'.  The rather thin blades of NEMA 5- and double the currents for the same wattage might suggest that they might be prone to rather different problems and failure modes than we're used to. I think shuttered versions of the NEMA outlets are available these days, but the US system still lacks the sleeved pins or recessed sockets of European practice, potentially allowing access to live parts via a partially inserted plug.

Don't assume that all US equipment is 115V or comes with NEMA-5-15 plugs - I've worked with American IT equipment in the past that came with 230V NEMA 6-15 plugs. While 15A NEMA 5-15 sockets are common domestically in the US, some of their offices seem to prefer the 20A NEMA 5-20 outlet (that can accept both 15A and 20A plugs). If it's modern IT equipment, you might find that it takes a IEC appliance ("kettle") connector (BS EN 60320) anyway - which would allow you to side-step a lot of the problems.

   - Andy.