The average coach in UK (the most likely 'bus' to be on motorway) is 12m long. Most motorway sliproads are required to be at least 200m long.
The bus must have been going quite a bit slower than OP at the time it pulled out for OP to have needed to brake to a standstill to make space for it to pull across. Which suggests that OP was effectively overtaking the bus as it moved down the sliproad.
If bus has speed limiter of 60mph (likely), this matches the OPs comment the bus was going 'at speed'.
But if the bus can only reach 60mph, and is trying to match its speed to the inside lane speed, what is it to do if a car at 70 starts closing the gap it has spent the last 150m lining itself up for?
Assuming OP was doing 70mph and was level with back of bus with 50m of sliproad left. That leaves:
- 4 bus lengths of sliproad left for the merge.
- At 60mph, this would be 2 seconds remaining on the sliproad for the bus.
- At 70mph, this would be 1.7 seconds for OP to clear the end of the sliproad.
- At 10mph faster than the bus, OP would be going 4 metres per second faster than the bus. So it would take her 3 seconds to get in front of bus, if she was level with back of it with 50m of sliproad to go.
- But bus runs out of sliproad in 2 seconds time, not 3 seconds.
- So what is bus to do? Well, obviously they have reached their maximum legal travelling speed in the 150m of sliproad so far, so should be able to enter a safe gap at matched traffic speed.
- If the inside lane was other HGVs with a legal maximum speed of 60mph, all would be well.
- But OP has rocked up, doing (presumably) 70, and not backing off because she has 'right of way' over traffic joining from the slip lane, which has a give way marking at the end of it.
- But if OP was doing 70 and bus was accelerating from 45 to 60 as it travelled down the sliproad, it would have been at least 2 bus lengths clear ahead of OP at the start of the sliproad, with a clear space beside it to merge into.
- If bus is already travelling at its maximum legal speed, with a merge gap beside it, highway code or not, it's got to be pretty life-threatening for OP to go into that merge gap at 70mph when bus isn't legally allowed to go any faster, if OP isn't prepared to move into an overtaking lane to overtake at that point.
If that's what happened OP, or any approximation of it, I'm afraid you're bonkers for soaking up all of the safe pulling out space the bus should have had in front of you at 60mph.
If you can't notice a bus moving down the slipway ahead of you & accelerating to 60mph, and that you are only just going faster than it - and your choice is to carry on and assume IT will stop rather than easing off the accelerator to slow to the maximum legal speed of an HGV joining the inside lane so you are safely behind it - then good luck for next time as well.
If you want to be able to proceed at 70mph no matter what, and not accommodate slipway traffic at their legal maximum, then you CERTAINLY need to be moving to the second lane ahead of the junction merge. You can't just assume slipway traffic will stop for you if they can't ever actually match your lane speed, no matter what the highway code says!
An opportunity for this to become a diagram thread...?