Viking Cruises offers both 110V and 220V outlets in your stateroom, with European style (2-pin) outlets with ground, except for two US 110V sockets above the desk and one on each side of the bed. There are no UK 3-pin sockets. You can purchase adapters at the onboard shop or electronics or travel stores.
You'll find a USB socket in your stateroom to charge cell phones and tablets, but remember to bring the appropriate cable. Most modern electronic devices don't require a current converter, except for appliances that produce heat such as curling irons. Hair dryers are provided on board. If you're staying in a hotel during your trip, you'll need the appropriate adapter for that country as well.