Tricky, there are now so many small deer that they have almost become a nuisance. One actually lives in my garden (the house is half a mile from Chesham town centre). It is a pest and eats all my runner beans when I try to grow them, but none the less it has been a 'resident' for about 3 years now and I would quite miss it if it left.
I too see them drowned in the canal along the Tring summit, sadly there is not much you can do to prevent this. If you are driving and hit one, it's generally an expensive repair to the car and if you try to rescue an injured one take care, they have small sharp horns, it's best to try to cover their head with a cloth to calm them down. The RSPCA round here (Chilterns) are not very helpful with injured animals, a local animal hospital St Tigglewinkles at Haddenham are far better.