As far as grass snakes are concerned, they are protected under the Wildlife and Countryside Act 1981 from being killed, injured or sold. If you catch one, you will be liable to prosecution.
Best thing to do if you want to keep snakes as pets is read all you can on them first...my son is reptile mad and now has 2 corn snakes, a 5 foot boa constrictor, a western hognose snake and a chameleon!!
He started with a baby corn snake (�40) as they are relatively easy to care for and handle, but he did loads of reading first, and spent ages talking to the guys in the reptile shop, then weeks persuading me to let him have one!!
There is also the cost of the vivarium, heating, lighting and accessories to consider.
Snakes in captivity eat mice and rats, but the bigger the snake, the bigger the food!! These rodents have been bred for the purpose and are bought frozen. They have to be thawed before feeding to the snake.
Snakes do bite at times, it's one of the hazards of keeping them!!
Hope this helps..