The Apple website states:

You should use the appropriate wattage power adapter for your Mac notebook. You can use a compatible higher wattage adapter without issue, but it won't make your computer charge faster or operate differently. Lower wattage adapters don't provide enough power.

The adapters are rated at a higher voltage and should be fine.

1) For your MacBook adapter: you need neither. Just simply buy a UK duckhead for your MacBook, already.

Either:

• the outrageously overpriced official "Apple World Travel Adapter set" (which is just five outrageously overpriced duckheads in a box), or
• (unofficial third-party) individual duckheads (you can't find these on Amazon, only on eBay, for (cough, cough) legal reasons). Yes, the UK duckhead will have an earth prong but probably won't be fused. So don't leave it plugged in overnight or unattended.

Context: I just got back from Europe with my MacBook, and before I went I spent the princely total of US$5.50 on UK + EU duckheads (+ $15 for the world travel adapter). I researched this heavily before I went. Apple egregiously abuses so-called "intellectual-property" laws to legally intimidate competitors from selling duckheads in the US, i.e. a piece of plastic with a few metal bits, to preserve their obscene margins.

2) For your phone charger, no you do not need a voltage converter, you just get an adapter (world travel adapter).

A neat and compact solution is a world travel adapter with built-in (twin) USB, better still if it's twin 2.2A USB; you can even get quad-USB if you have lots of devices. These are the ones with multi-type A/B/C/G with quick-slide retractable prongs, they're lightweight, compact and retractable so they fit nicely into carry-on baggage, good for a quick use at the boarding gate, cafe etc. Ceptics is a superb brand. Just be gentle with the mechanism when you retract one type of prong and extend the other.

As to the ambiguities in the phrasing of your question: if "device" = laptop then no you don't need a voltage converter (but do obviously still need an adapter); laptops have universal voltage converters. Only if "device" = shaver/kettle/heater/desktop/other power appliance might you need an actual voltage converter. But that wasn't what the question asked about.

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.