Princess - If your electronics say "100-240V 50-60 Hz" then all you need to do is convert the plug shape. Note that some electronics might be rated 100-220V or 230 instead of 240 - these should also be fine.

Note that most electronics - computers, phone chargers, etc - sold in the last few years are 100-240 V. Things like hair dryers and curling irons tend to have a switch you need to set to the correct voltage level, but this feature isn't present on all hair dryers/curling irons. If your hairdryer/iron has a 120/240V switch then all you need to do is convert the plug shape. Else you need a transformer that works with these items. These sorts of transformers tend to be heavy and large because they need to pull 10-15 amps at 120 volts. The pocket size transformers for electronics will not work for high power devices like hair dryers. If your hair dryer/iron don't have the switch, it's probably better to just go get a "travel" one that does and is small. It will probably cost no more than the appropriate transformer, and weigh a lot less.

If you're staying in a halfway decent hotel in the UK, it will most likely have a hair dryer in the room. Don't know about Russia.