charger unit is rated 120volts
Check the label on your charging unit, under the heading of "input"
It should list a voltage input in AC, typically 110-240 VAC (that's volts AC). If it says that, then your charger can handle UK voltage and you just need an plug adapter on the end. If it says something different (like 110-120 VAC), then you'll need a transformer to move the 220v UK voltage to 120 volts US voltage.
Actually the Nikon charger is rated "AC100-240v, 50-60Hz"
All you need is a plug adapter … no transformer is needed.
I used mine in Africa (250v 50Hz) and when we had electricity it worked fine.
References :
ProFotog