How can I provide power to a dehumidifier in the bathroom?
The important rule about bathroom electrics is that, apart from shaver sockets, fundamentally you can't have any power sockets in there. There may be some exceptions, but they don't apply here.

However, you can have power going into a bathroom if you hard wire the device and have a switch outside the bathroom that can turn the device off (we have a couple of heated towel rails wired this way).

So as long as your de-humidifier isn't directly above the sink, bath or shower and wired this way it should be OK.

However, with all these things consult an electrician. Even if you do end up doing it yourself getting professional advice is always a good idea.