I use precision engineered whey protein from Holland and Barrett. It cost me around £40 for 908g, but it was in the penny sale, so I got a second 908g tub for 1p.
In one serving there is 84kcal, 17.8g protein, 0.2g sugar and 0.8g fat. It's not the highest protein but it was convenient to buy that one when I was passing the shop! I didn't use it pre-op as it didn't occur to me.