There's some incorrect information on this thread (along with some good advice).
To be clear...
Corn snakes are non-venomous and completely harmless - to adults, cats, babies... pretty much anything except mice! Your cat will certainly hurt the snake, though.
They are native to the US, not the UK. They are used to higher temperatures than we usually have here. It is an escaped pet.
They are largely nocturnal. It is more likely to come out at night, seeking warmth. It is extremely unlikely to approach you and will try to get away from you if it can. If it's cold, it will move more slowly.
The best advice you've had is the mouse/water bottle. How big is the snake? Go to your nearest Pets at Home or pet shop and get a frozen mouse appropriate to the size of the snake (if you tell me them - or me - the size of the snake, they'll tell you what size to get). Follow the instructions from the PP upthread - don't forget to punch airholes in it. If the mouse is too big to go through the neck of the bottle (which hopefully it will be, as then if the snake eats it it will be trapped in the bottle), but the bottle in half, put the mouse in and then tape it up. If you can, put just one radiator on low overnight, to attract the snake to the heat. If not, fill a hot water bottle before you go to bed, wrap it in a towel and put the bottle beside it. Drape the bottle with a pillow case put a few cushions around it - something to make it seem safe and cave-like to the snake.
That will probably attract it if it's in your house. Not much point if you don't think it's in the house though, obviously. You could try the lines of flour/cornflour tonight to check that first.
I wouldn't recommend that a snake-phobic sit in a dark room waiting for the plastic-bag rustle of a snake approaching though! 
If you catch it in the bottle, then tape the end of the bottle over (make sure you put air holes in the bottle before baiting it though, obviously) and either the RSPCA will collect it, or you could take it to a pet shop. They're very popular pets.
Definitely check local facebook groups to see if anyone's lost it (in which case they'll come and try to tempt/attract it to be caught) or whether there are local snakey people who can help you.
I know it's awful when you have a phobia, but please try to keep in mind that this is a living creature, someone's loved pet, and is totally harmless to you. It's just trying to find somewhere warm and safe.