Nobody seems to have mentioned the "safe"? alternative way of joining a motorway.
At start of sliproad, floor accelerator until doing at least 100MPH so everything will be slower. Aim for gaps in traffic and launch oneself immediately into outside lane, then slow down to match the 85mph fixed speed traffic for rest of journey.
When the time comes to exit motorway, repeat procedure, making sure there is at least 3 inches of lane/zevrons left before cutting across the car in the way.
I see this lot's, so it must be the safer/preferred way?
Neil