Gobsmacked at the number of people quoting the highway code.
Yes it sounds like the bus was not adhering to the code. The fact that so many posters are making a big thing of this means they don't do much driving on busy motorways.
HGVs, PSVs (and most drivers with go faster stripes or equivalent) on slip roads expect left Lane traffic to accommodate them. That's the way it is.
Anyone who doesn't allow for this will end up getting annoyed or frightened as per op.
The majority of junctions have an off ramp - the least stressful way of dealing with an on ramp is to use that as the flag to move out of the left Lane and move back after the on-ramp merging traffic has settled down - you always get people moving out of the left lane, either as they join or just after - so let them do this, chill.
There are some junctions, always well signposted, with only an on ramp. If you really can't get into the next Lane before the on ramp then adjust your speed to give plenty of space in front of you - that then gives you scope to match speed with the merging traffic.
(HGVs will usually be entering at a maximum -56mph, PSVs at upto 70mph/100kph)
If you want to carry on quoting the highway code - feel free - but if you don't change your driving behaviour you'll get the same thing happening again.