If you are buying potatoes this time of year, you are right in between new and old potato seasons. So you have a couple of choices.. There are some very good imported Maris Piper about at the minute and some very good imported Marfona.
If you are buying old crop potatoes, (which incidentally there is nothing wrong with them) you should be looking for Maris Piper, Cara, King Edward or any of the other High Dry Matter Potatoes. These will give you that nice dry, fluffy texture.
Avoid Harmony, Nadine, Kestrel and some of the other generic white potatoes. These are grown and packed for yield