Very good advice about packing the owners manual.
Any recordings and a press pack might be handy too.
Even on domestic flights, they get really crazy about any transitorized device the particular insprctor hasn't seen before.
Even without a theremin you need more time now then ever. You'd think they would have gotten much better at this by now.
That's great news if you do not need a transformer, just a plug adapter.
I guess it depends on the electical item itself.
Found general facts, pretty much what Gordon said, but very little about specific electronic devices. What seemed mildly useful I'll paste below.
bon voyage!
"Electricity
The voltage used in Britain is 240 Volts AC at 50HZ. Most power sockets are designed for standard 3-pin square plugs. Electrical appliances in Britain generally use the British standard plug with 3 square pins. Plug socket adaptors and power transformers are widely available, you can buy them at most airports, electrical shops and hardware stores."
http://www.visitbritain.ca/about-britain/practical-information/utilities-and-services/
"Electricity
Britain runs on 240 volt (50Hz) electric current. North Americans will need a transformer and an adapter for appliances such as electric razors, hair dryers, etc. Some hotels supply hair dryers in the bathroom.
"
http://www.britannia.com/PriorityTravel/travel_resources.html
Primer on "Electricity in Europe"
http://www.enjoy-europe.com/hte/chap11/electric.htm