This is what the RSA says:
"- As you approach the motorway on the slip road, check your mirrors and your blind spot for a safe gap in the traffic.
- Give way to traffic already on the motorway.
'
Change your speed to fit safely and legally into the traffic (lane1). Stay in the slip road if it continues as an extra lane on the motorway. Avoid crossing a solid white line that separates the traffic lanes. "
Source
So, you are required to adjust your speed to fit into the traffic of lane one. You can't do that safely on a busy motorway like the M50 without having the same speed as lane 1.