There is no voltage input switch, since the TC will operate on 100-240 Volts and 50-60 Hz automatically.
You will likely need an adapter to let the U.K. plug work in a North American AC jack, but you will not need a voltage converter.
UK models of the TC do not offer higher 5 GHz channel capability, so you will be limited to the more crowded lower 5 GHz channels here.
Personally, I would not purchase a UK product for use in North America, especially a Time Capsule, but it is your decision.
Apple just introduced a new Time Capsule yesterday. You might want to take a look.
Apple also offers refurbished Time Capsules....with new warranty......through the online Apple Store.
As Bob commented, all the apple routers are sold with a region code in the part number. There is no way to defeat this. That limits or makes certain bands or frequencies unavailable or ones that are illegal available.
European models use ETSI which is very different to the FCC setting on the US model. They can certainly be used.. but it is better to buy in the market you intend to use it in.
You can use it... US is more restricted at 2.4ghz than UK.
But 5ghz ???.
If you buy the older N wireless model N wireless was only really same room or next room.. It has very poor range.
The new AC one you do need 5ghz.. all the goodies for AC wireless are in the 5ghz band.
Look at the different bands.
For most people it won't matter.. because you will be using 2.4ghz and US is a subset of UK.. not the other way around. But you should really be looking at great financial advantage to do so.. otherwise buy UK.
If you could just change firmware or just set UK as region.. all would be good. But you cannot!!
Australia.. Colonial outpost and UK gulag.. follows UK.. and hence European standards for most things (the roof of the Melbourne Main Train Station can withstand 5' of snow) and lots of people buy apple routers in the US and use them here.. it is no biggie.
But recognise all is not straight forward in the world of wireless regulations.
I can get a brand new one from Apple for £240 and get a warranty. I've just been looking at the new ones with the latest spec http://store.apple.com/uk/product/ME177/airport-time-capsule-2tb & wondering if one would work with my 2012 MBPR and a 2006 iMac??