In Switzerland I did it 2 weeks ago without any problems for long periods. It was fantastic and I felt like a kid again. The coach behind us had Japanese tourists who were leaning way out. I didn`t see much danger and to be honest where objects did get close it was a case of haul yourself back in. Having said that the trains weren`t travelling anywhere near 125 mph. Obviously not wise to do this in the UK especially at speed but there are many youtubers and some of which are members here who shove their cameras out of windows and this seems acceptable. Why are some seen as low life scum and others not it seems. Don`t get me wrong. I`m not advocating this type of of thing in the UK for sure but let`s be honest. Only the completely stupid would get their heads knocked off and they would stick their heads in a meat mincer anyway.
When earlier in this thread I criticised someone for having a go at at platform staff for telling them to get their arm in, I deliberately didn't say what I wanted to, it felt like it wasn't needed. Seems it is though.
Don't ever hang out of a window of a train on the national network. If you really want to, go to perhaps the Snowdon Railway or the Romney Hythe and Dymchurch. But still be careful. The speed is irrelevant for whether it's safe or not. Make the most of an HST vestibule (inside) at 125mph if you haven't while you can though!
Why do I care about this? Because I remember in the 80s when an enthusiast was killed by looking the wrong way going into the Conwy tubular bridge (I'm from Conwy). We were horrified by it. I can't find anything on the internet about it, but it did happen (the internet often doesn't know anything about before it existed!)
Be careful and be safe, I know some of that might have sounded a bit patronising, but it's completely not meant to.