Not all plungers are created equal. In the UK, you want a (the one with a simple rubber cup), not a flange plunger (which is for sinks).
Before you do anything, locate the isolation valve on the pipe feeding your toilet (usually a small valve with a slot screw head) and turn it 90 degrees. This prevents the cistern from refilling and accidentally flooding the bathroom if the blockage shifts. how to unblock a toilet uk
If you have these kitchen staples, they create a fizzy reaction that can agitate blockages. Not all plungers are created equal