What does a motorway symbol with a diagonal bar mean?
This marks the end of the motorway, where motorway rules stop applying.

A sign showing the motorway symbol crossed by a red diagonal bar, sometimes with the distance to the end.
What you must do
Be ready for a different speed limit and for junctions, oncoming traffic and slower road users on the road ahead.
Where you will see it
Where a motorway ends and joins an ordinary road.
Test yourself
The theory test asks you to recognise signs like this one. Try the road and traffic signs short test (with an easy and a hard set), or read the signs revision notes. Source: Know Your Traffic Signs.
Other common road signs
Meaning based on Know Your Traffic Signs and The Highway Code. The sign image is an official Department for Transport sign (Crown copyright) from Wikimedia Commons, reused under the Open Government Licence or the public domain. Always check the current Highway Code on gov.uk. Not affiliated with or endorsed by the DVSA.