Two things change when you fly from the United States to Britain: the shape of the plug, and the voltage. Only the first is solved by an adapter.
The plug
Britain uses Type G: three rectangular pins in a triangle, with a fuse inside the plug and shutters in the socket that only open when the longer earth pin goes in. Nothing else fits. You need a US to UK adapter — sometimes sold as "Type A/B to G".
The voltage
The UK supplies 230 V at 50 Hz. The US supplies 120 V at 60 Hz. That is a different voltage class, and an adapter does nothing about it.
| Device | Marking to look for | What you need |
|---|---|---|
| Phone, laptop, tablet, camera | 100–240 V, 50/60 Hz | Adapter only |
| Electric toothbrush, shaver | 100–240 V | Adapter only (most hotels also have a shaver socket) |
| Hairdryer, curling iron | 120 V only | Adapter + step-up converter, or buy locally |
| CPAP machine | Check the label — many are dual voltage | Adapter, plus a converter if not |
Hotel sockets
British hotels usually provide a two-pin "shaver socket" in the bathroom, which accepts US and European two-pin plugs. It is limited to a few hundred watts and is meant for shavers and toothbrushes — it will not run a hairdryer and should not be used for a laptop.
The fuse question
British plugs contain a fuse, usually 3 A for small electronics and 13 A for appliances. Adapters are sold with a 13 A fuse and are fine for chargers. If you are plugging in something with a heating element through an adapter, check the fuse rating first — that is the second reason a US hairdryer in Britain is a bad idea.