I disagree with the OP.
There may have been a reason why the lorry couldn't get up to more than 40 on the slip-road. Was it an uphill slip-road? Was the lorry heavily laden? Was there some other vehicle in front holding him up that you couldn't see?
Whatever the reason, you should have joined the motorway at the appropriate time, even if that then meant being 'stuck' behind the lorry for a few more precious seconds and showed a bit more PATIENCE before asserting your 'right' to get out and hog the middle or outer lane for the rest of your journey.
Chill, man!
Jeremy, I am the OP, the driver of over 20 different cars, over the 30 years I have been driving. In that time I have learned to tolerate a great deal of insanity with regard to the roads and the road network in general. I have every patience when driving, and believe that I have a very good grasp by now, of the rules of the road, both written and unwritten. When driving on a motorway, you drive at the speed of the flow. the speed limit on the motorway network is 70mph, (with the exception of HGV's, Coaches, Buses etc are limited by law, to a speed that is safe for that vehicle). The vast majority of cars on these roads are capable of far in excess of 70mph, but again are limited to this speed for your safety. There is a fine network of A roads in this country, they were the equivalent of the motorway network in my fathers time, if you prefer to drive at 40mph, then stick to these roads, if you find it frightening to drive on the motorway, then don't go on it. Its ok to be scared of entering a motorway at any more than 40mph, all those big trucks will slow down and let you in, they won't struggle to get past you, at thier limited mph, because you found the strength to get into third gear and hit 50, they won't cause any problems whatsoever in your wake, as they pass you and box you in, holding up the freeflowing traffic behind them, they wont cause the ubiquitous, lane two hogger ( doing 60mph in the second lane) to move into the outside lane (still at 60, I might add) and slow down the third lane traffic, to a crawl, and the ripple effect of progressive braking as the oncoming traffic struggles to fit into an ever decreasing space, massive traffic chaos, and even accidents. Is this the type of motorway network YOU want, all because its ok, and I should chill out. Listen sunshine, the motorway network was designed and built to get Goods and Services, People and thier families, to where they want to go, as quickly and efficiently and safely as is possible. It IS NOT the scenic route to Tesco, NOR is it your own personal drag strip. Take a moment and read through all the posts on this thread, there is some quite inspired reading there. It is not my intention to belittle you in any way shape or form, but DO NOT EVER tell me to CHILL OUT, when I post a thread about the morons who join the motorway network at stupifyingly slow speeds, leaving in thier wake, utter carnage with the potential of causing very real and devastating accidents.
Each and every person with a full driving licence in this country has rights, under law, to drive on the motorway. My point is that not everyone has the moral right, it is not a question of whether they CAN, it is a question of whether they SHOULD !!!, when they can get to exactly the same places via A + B roads. !!!!!
p.s. It wasn't a lorry, it was a car, quite a powerful 3 series BMW actually, driver on the phone, and on a downhill slip-road stretching about a quarter of a mile, junction 6 M61, just the other day.