Look at the device. If it says 120-240V, you're all set
It may say a wider range like 100-240V (or 90-264V which is that +/-10%). 100V comes from Japan, if you're wondering.
Your Apple branded chargers will all be multi-voltage. Third party chargers may vary, but in all probability, they're multi-voltage too because of the magic of switching power supplies.
Since they are multi-voltage, all you need is an adapter to physically connect the US prongs to the UK socket. This device will contain only copper and plastic. Beware cheap Cheese junk; look for the mark of competent testing labs such as UL, CSA, BSI, TUV, ETL, etc.
Most things care about voltage. Some things also care about frequency. So check each device individualy.
Any run-of-the-mill IEC C7 cord will also do
You've seen C7. It's is the little socket used on everyone else's laptop power supplies, tape recorders, projectors etc.
On some Apple adapters, you can remove the plug portion. That exploses an inlet* for IEC C7. You've seen C7 before. You can use any run-of-the-mill IEC C7 cord, or you can get different Apple plug modules for different countries that snap in just like the original, if you don't want to drag a cord around.
The standard, bulky UK socket is called a BS1363
It's much more productive to search for "BS1363" than "G". So for instance you might search for a C7 to BS1363 cord.
Note a couple unique things about BS1363 that differ from the compact US sockets:
- The BS1363 socket has a switch on it. If the socket is dead, try the switch.
- The BS1363 plug has a fuse in it. Yeah. Seriously. So if nothing else is working, check the fuse.
* Technically speaking, an "inlet for IEC C7" is C8, but let's not confuse the matter. C7 is what you need.