Not part of the answer: I've considered doing the same thing, because 240V kettles rock. However since installing an induction cooktop I've had far less reason to do it, as induction does such a good job of boiling water quickly.

@jay613 great answer and suggestions, thanks very much. that duplex receptacle looks like exactly the right thing. this is all motivated by an upcoming bit of remodeling in the kitchen so the walls will be open for ease of running /3 cable.

I think if you use the combo outlet you can even put it on a 20A breaker.

The underlying issue, BTW, is that nobody has bothered to trundle a bunch of their BS 1363 receptacles off to UL to get tested and listed to the UL 498 standard that governs receptacles in the USA. That said, why would they? We have NEMA 6 for the job, as the answer points out, and foreign appliances can simply be retrofitted with NEMA 6 plugs for US use.

That's one issue, but I don't think it's the only one. Other obvious ones that come to mind are the unusual current rating (which is an issue as the NEC restricts what current ratings of outlets can be installed on a given circuit) and the fact that british accessories don't fit american electrical boxes.

Note that in the UK, plugs contain fuses, the kettle plug will contain a 13A one. Replace with a US plug, and you don't have that anymore. Depending on the age and design of the kettle, there may or may not be another fuse inside it. If there is (common in kettles with detachable cables), then you're just losing proper protection of the cable - which is considered "safe enough" in the USA. If there isn't, you should probably find a way to have an appropriate fuse or breaker somewhere else - I'm not American so I don't know if that is implicit in the use of a 6-15 socket.

I deleted my earlier comments. Since you're opening walls and running new cables, and since the kettle will use almost 13A when running, you are better off running two /2 cables to a new two-gang box, use a single Decora 6-15 outlet for the kettle and a GFCI outlet on the other cable for 120V. You get better selection of outlets in the right style and color and you can plug anything you want into the 120V outlet even when the kettle is on. With a 15A breaker you don't have to worry about eliminating the 13A fused plug. Use #12 cables in the walls for flexibility in future.

The dual voltage outlet solves problems you don't have, and creates new ones.

@JackB If I'm not mistaken, fuses in UK plugs have to do with protection related to usage in ring circuits. The same appliance in other European countries with type E/F plugs won't have that fuse, since they're not using ring mains. There might be other considerations in the USA, but I'd assume they're not using ring mains either

@Bruno The fuse in the plug is required because the ring main feeding the socket can supply 30+A but the cable is only rated for 3-13A (and the fuse matches the cable). You also usually need a fuse somewhere in the system to protect other things. If the device is for sale in worldwide, it will have a fuse inside so it can be used with any plug, and both line and neutral should be fused in case that plug is reversable. If the device was only ever sold in the UK, it may only have a fuse in the plug, and that fuse protects both cable and device internals.

There are lots of devices sold in the US that have an 18-gauge cord, no fuse, and get plugged in to a 20 amp circuit, and it works out fine. Yes there's some risk to running the device without a fuse in the plug, but I wouldn't lose sleep over it.

@KMJ: Such devices are required by UL/ETL/etc. to be designed with something internal to the device that would interrupt current under any plausible scenario that would otherwise cause the device to draw too much current. A kettle designed for operation from UK mains might not.

UL/ETL listed devices are, sure. Lots of items people buy from discount stores and Amazon are not UL listed though. The 'items that plug in to the wall' world is not exactly tightly regulated.

Re, "you cannot legally install a British receptacle in your US house." I used to work in a light industrial building in the U.S.A., and we had British power outlets all over the place. We had a motor generator set about the size of a small truck that provided them all with actual, 50Hz, 230V power. IDK whether it was OK because we got some kind of variance, or OK because it wasn't a residence, or maybe somebody bribed the electricians and/or the building inspector. Maybe it was OK because it was the 1980s and they let you get away with stuff back then that you can't get away with today.

I suspect it's actually fine and because OP has an actual use case and it's just going to be a bit more trouble than in the permit process than usual. The electrical code is not intended to limit safe household appliances.

@SolomonSlow you can do a lot in engineered systems, so it was probably that. Or the inspector just didn't care.

@KevinCathcart if that comes up, you installed it for your toaster amazon.com/Waring-Commercial-WCT855-Toaster-Silver/dp/… which is UL listed. You just haven't ordered it yet and are installing the outlet in advance. I think an AHJ that actually wanted to apply 422.6 to a plug in item isn't going to stick around after you change out the toaster for a kettle.