Yes, Black Prince does have a bureau de change, but you'll get far better rates in the UK. Try M&S: no commission and very competitive exchange rates. You can order online or over the phone. For over-the-counter, the Post Office is still good.
Yes you can. The reception desk generally holds currency for the ports of call relevent to the cruise you are on. hope that helps.
The reception desk on board holds a stock of currency for each port of call. Best to get there early though as it can be very popular, and you don't want to fall short.