It would help to know where you are in the UK. In my area - I'm not far from an excellent retailer (in Darlington) who can probably get hold of most species. Your best option would be to check reptile shops in your area, or publications such as Exchange & Mart, or other trade papers. I would say never buy any animal without seeing it 'in person' first. If you buy something without viewing it first, there's no guarantee it will be a healthy specimen.
It depends where you are. You can't buy one in the UK - as The European Grass Snake (Natrix natrix) is protected under UK conservation laws. It's illegal to take them from the wild - and there are unlikely to be any for sale in UK pet shops.
The only venomous snake native to the UK is the Adder.
There is quite a few species/breeds of snake in the UK, but the 'Adder' is the UK's only venemous snake.
That would depend on how far out to sea you are off the coast of the UK.
The Yellow Bellied Black Snake of Australia a close relative of the Red Bellied Black Snake.
A young common grass snake
Legally - you DON'T ! ALL UK reptiles - the Adder, the Smooth Snake (which is actually a lizard) and the Grass Snake are now PROTECTED BY LAW. There are severe penalties for interfering with an of the UK's naturally occurring reptiles.
A Snake in the Grass - 1903 was released on: UK: 1903 USA: 1905
Black Adder
My locak brancj of Pet Mania does a corn snake starter kit complete with snake for around £100
They do in the UK - it gets too cold during winter for them to function effectively, so they seek out sheltered caves and other warmer places to 'ride out' the cold weather.
In the UK, if you need the Coast Guard in an emergency, dial 999 and ask the operator for the Coast Guard.