"The NHS said about 100 cases of adder bites are reported in the UK every year.
Since records began in 1876 only 14 people have been reported to have died from adder bites - the last in 1975."
Source: http://www.bbc.co.uk/news/uk-england-hampshire-13177046
Nasty but not that much to worry about.
Being shot isn't necessarily fatal either. But you really need to worry about it if you are, and get medical treatment immediately. The proteins that make up adder venom are very complicated. They contain neurotoxins, haematoxins and cytotoxins. A very nasty little mixture that is.