You may want to have a look of my review. it may help.
http://www.cruisingtalk.com/p-o-crui...bean-j625.html
As for position of cabins. Midship certainly does have less movement. But then I don't think it really makes a difference on the new ships.
AS for where to get the best prices all I would suggest is go to the P&O website get a price from them and then try a few travel agents either on the high street or the web and compare prices. You should be able to get the price knocked down a bit that way.