Voltage converter

http://www.ricksteves.com/travel-tips/phones-tech/electric-adapters-converters

Note that "converters" ususlly mean devices that transform 220-240v 50 Hz electric current to to power a North American device designed for 110-120v 60 Hz. Adaptors allow NA plugs to fit non NA electric sockets. See RS note re dual rated electronics.

Heating devices e g irons can be a problem if not dual voltage or 240v rated. And if they are check the voltage switch position.

Just to echo that, most common devices (phones, laptops, tablets) need only an adapter these days, rather than a converter. Pick the smallest available, really. If you are only going to the UK, you don't need multiple solutions and there is very little difference in the various choices. My multi-country is a very small, well packaged set from Sharper Image brand but you probably can't find it any longer.

Butch,

You'll first need to determine whether you actually need a Voltage Converter. Could you elaborate on the electrical devices you plan on travelling with? There are two principles used in voltage conversion, and it's important to match the correct type with each device.

Regardless of the Voltage Converter issue, you will need UK Plug Adaptors. I'd suggest packing at least two.

Why you should not bring a voltage converter.
http://www.enjoy-europe.com/hte/chap11/converter.htm

Just to expand on harleydonski's note about voltage transformers vs. half wave converters, a transformer capable of handling enough current (amperage) for heating devices is big and heavy. My 240 v waxing iron is rated at 800 watts. I saw a similar but 120 v iron fried in Germany when the "converter" didn't do its job. Here is what a 1000 watt stepdown transformer looks like:
http://voltage-converter-transformers.com/collections/step-down-voltage-transformer-converter/products/vod-1000-1000-watt-step-down-voltage-transformers-ce-certified

Note the transformer's description:
Weight : 18 lbs
Dimension : 8" X 8" X 5"
This is not a carryon device.