You have to be careful because some people mix up the terms "adapter" and "converter". If the word "plug" is specifically used, then I would say they mean the little plastic and metal gizmos that just go between a US plug and a UK outlet, which I call "adapters". These do not change the voltage. They cost about $5 or less.
This is described as a "US to UK Plug Travel Adaptor" £2.89 on Amazon
The heavier, more expensive things in a case that have a cord with a UK plug on the end, and a US outlet on the case, that change the UK 240 volts to US 110 volts, I call "converters". These have a power rating in Watts, the bigger this number is, the more they cost. This first picture is a 150 Watt laptop adapter that costs around $20.
The second picture is a big, beefy 3000 Watt adapter that could run more than one power hungry item like an iron, microwave, etc, but would be HEAVY (no weight mentioned, but I would be surprised if it weighs less than 15 lbs) and costs around $100 upwards.
If your laptop is multi voltage you just need the plug adapter, or you can just buy a new cord when you get here.