Since you can't have a mains-powered device in a bathroom (unless you pay hundreds of pounds to have dehumidifier fan installed), you'll need something like this: http://www.clifford-james.co.uk/online.cfm/home-and-kitchen/cordless-and-reusable-compact-dehumidifier/68/no/66609
Putting a dehumidifier in a bathroom is pretty pointless . There is the moisture from the sink, bath,shower, damp towels so there is always going to be a virtually limitless source of moisture. More information as to why you need a dehumidifier ?