Don't be afraid of pitching an attentuative yeast for a milk stout. (5 gallon batch) I added 1 lb of lactose ( at 10 mins left) , and pitched SafBrew S-33, highly attentuative and still ended at 1.030. A nice sweet, milk stout. I would actually be careful about a Whitbread yeast, possibly leaving it too high, maybe? Pitch a yeast with good attentuation, don't let the OG fool you. It'll still finish high, the lactose is not devoured by the yeast.