Turn off the water at the isolation valve behind or beside the toilet first — that's a small chrome or plastic tap on the pipe leading into the cistern. Don't flush again to try to clear it; that almost always makes it worse.
Find the isolation valve
Most toilets have one on the supply pipe low down behind the pan or cistern. Turn it clockwise (or push the tab in on push-style valves) to shut off the water feeding the cistern. If you can't find one or it won't turn, the stopcock for the whole property — usually under the kitchen sink or near the front of the house — will do the same job for the entire property.
Why it's overflowing in the first place
A single toilet overflowing on its own is usually a blockage in the toilet's own trap or the short run of pipe just after it. If other drains in the house are also backing up — a bath or sink downstairs, for example — that points to a blockage further down the shared drainage system, which is a bigger job and needs sorting properly, not just cleared at the toilet end.
What not to do
Don't keep flushing to try to force it through, and don't pour chemical drain cleaner into an already-overflowing toilet — it won't reach the actual blockage and just adds a hazard to a wet floor. Stop the water, contain what's already out with old towels, and call.
When to call
Water on the floor, more than one fixture affected, or a smell of sewage — these mean it needs sorting today, not booked in for next week. Call 07468 868807 and describe what's happening; you'll get a straight answer on how quickly someone can get to you.