Ceiling light won't turn off
It is likely the switch needs replacing - probably the switch contacts have welded together due to arcing.

If you can easily and safely remove the lightbulb, that will stop current flowing through the switch. As I'm sure you know, bulbs - especially the older incandescent tungsten-filament type - can get hot enough to burn you. 

If you cannot access the light bulb, so long as the switch is not buzzing, smoking or emitting a burnt smell, it is probably not in any immediate danger of catching fire.

If you have easy access to the circuit breakers in the consumer-unit (fuse-box or main-panel), you might be able to turn off the breaker that controls the circuit for that switch and light. For peace of mind I would do this before going to sleep. In a Welsh house there is probably one circuit-breaker for all the lights on the ground floor. Your upstairs lights will still work normally. I would take a torch with me and check the torch works before switching any circuit-breakers off.

If you are really worried, you may be able to get assistance from your local fire-department on a non-emergency basis.