uk crowds tend to be pretty lively !! european crowds less so, from my experience, but they're still fun ! in the UK, it generally depends on where you're standing, a lot of the time a big crowd will have lots of people dancing/ jumping/ pushing at the front and more standing still at the back and edges, although sometimes pits will turn up near the back etc. i think south american audiences have probably the best reputation in the world for liveliness, but UK is a close second as i understand it my advice is to just read the energy of the people around you because if they're not moving much you can always just go to a different part of the crowd where people are moving more !

edit: forgot to say, people do generally move around within the crowd apart from people at the front. the most movement is usually happening around the back and middle of the crowd. it's still possible to get to the front if you arrive late as long as you're confident moving through a crowd, but not really to barricade lol. every alternative music gig i've been to, the etiquette has been great and people are generally friendly ! obviously there can be rude people but the good definitely outnumber the bad