What Car? Q&A - Who has right of way at slip roads?
A: Vehicles already using the motorway have the right of way, so you shouldn't brake or slow down as you approach a slip road - it can be dangerous to do so.
If you're in the first lane, it's a much better idea to look well ahead and move over if you can see traffic that wants to join the motorway.
Don't move over if this would force other motorway drivers to change their speed or position, however.
Ultimately, it is up to the vehicle joining the motorway to give way, if necessary, and filter on without causing any trouble.
The Highway Code says that traffic joining the motorway should 'give priority to traffic already on the motorway' and 'adjust speed to fit safely into the traffic flow in the left-hand lane'.