Unless specifically designed, like this waterproof rucksack from Aquapac, most packs designed for walking and trekking are not waterproof. There are some that are better than others and some brands and models claim better water resistance but in prolonged glorious British weather then you'll need to plan carefully to keep kit dry.
Most rucksacks have a highly water-resistant treatment applied to the outside but will begin letting water in during heavy downpours. Lots of our range come with a with a rain cover, perfect for pulling over your bag when the heavens open, so check out the spec to see if the model you are looking at includes one.
Ultimately the best option is to use a rucksack rain cover if needs be and then a rucksack liner, a flexible roll top bag, sometimes knows as a "dry bag" available in multiple sizes, that goes inside and keeps your gear dry, whatever the weather.