get the cheapest, or, do the price match as it usually shows what options you can get for a reduced cost, as recommended above try thr price match first, if nothings there then the impact whey protein, its listed as something like £17ish for 5lbs, 30ish for the 10lbs option though the price matching should knock a couple quid off that.
I'd order a smaller amount, I'm(i think) slightly lactose intolerant and the isolate(which has less lactose and is basically more filtered) version which is quite a bit more expensive, leaves me feeling less phlegmy and mank after it, I also get that way when I drink milk. But even so I'd recommend a smaller purchase to start you off on the off chance you can't stand it.
I use cheap flavourings myself for the stuff, cocoa powder is cheap(2-3 quid for a quality tub of the best stuff and will do 100's of shakes) and a cheapo pack of sucralose and I'm sorted, add's 2-3p to the price of a shake probably. Loads of people use nesquik though you're essentially just adding sugar, which may or may not be a problem for you.