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.