It's nobbish to not let someone on to a slip road when you can do
Depends what you mean by "when you can do". And I assume you mean "off a sliproad onto a dual carriageway/motorway"?
When joining a motorway, the onus is 100% on the joiner to join safely. That is because it is very dangerous for traffic on the motorway to have to organise themselves around the joiner.
Despite generally advocating a "let people in if you can" approach here, I consider a motorway the one time you do not normally consider slowing down, either naturally or worse, by applying your brakes, to let someone join. They must speed up or slow down to match the speed of the carriageway, then they can pull on without anyone needing to do anything.
I do not want you to do anything unexpected to make it easier for me, because I consider by doing so, you are making things more dangerous for yourself. I am perfectly capable of getting onto a motorway with no input from others, even if people haven't moved across in advance.
If there is conflict, I, on the sliproad, will brake. It is safer for me to brake than for you to brake. If one of us is going to get rear-ended, it should be me. It is the best outcome in an impossible situation.
Usual disclaimer that motorways don't always operate perfectly and sometimes sliproads are very short, are more like merges than sliproads etc. And always an idiot who doesn't check the carriageway until the last minute and has to emergency stop at the end of a sliproad blah blah. I'm taking in ideals here.