I have seen 2 snakes in the wild in my whole life. One of which was a very large adder, which did not hang around for very long!
Who ever told you what you have just said is very confused indeed! I the UK there are a few types of snakes, some fall under the grass snake catogrey which are complety harmless.
This person that told you there are adders and vipers is wrong because the adder is a viper, the viper is a family of snake, some vipers such as the nose horned viper are extremly venemous and can kill you (these are not found in the UK) the adder is not very venemous at all and if it where to bite you, it would not cause anything more then a slight iritation.
I doubt you will see any snakes unless oyur looking for them, and can guarntee that unless you try and catch one there is not the slightest chance of getting biten.