There isn't a known issue with signal at the stadium, it's always been fine when I've gone there. Which network out of interest? The Bowl always had a mobile phone problem because of the issue with there being 70K mobile phones in an area (Furzton) that has a population of a couple of thousand normally, but the Stadium is normally used for crowds!
Signal strength itself seemed fine but data just wasn't really happening. Enough to maybe get a WhatsApp message sent or send a Tweet but good luck actually looking at your Twitter feed or looking up train times etc. Seemed to be across all networks. Certainly both my EE and O2 sim's were acting in the same way, as was my partners Giffgaff sim. And the comments on social media seemed to confirm that too. I suspect it is just the sheer number of people trying to use data. MK Dons get no where near that number of being at their games most of the time (their average this year was about 9k I believe, this was at least double if not probably triple that number), and as far as I am aware the stadium hasn't actually held many gigs of a similar size before anyway (I think the 3 MCR gigs this weekend would have been gigs 5, 6 and 7 that the stadium has held). Switching down to 3G did help at times (I guess just less people using that particular part of the network - same trick I've used at football matches before) but there were points of the evening where you just couldn't do anything that relied on data regardless of what you did (and I wouldn't expect "normal" people to have the know how or even have the idea to switch down to 3G to try to get data!).
Prebooked taxis are not really a thing anywhere; all it means is that you've got a booking in the system which they'll allocate to a driver if they have one based on the current waiting time-ish. People do need to better understand what prebooking a taxi actually means.
That certainly isn't my experience but saying that I have only used prebooked taxi's in more rural areas before where its more like one man band kind of deals so there aren't really drivers to allocate! I know Uber and other ride sharing apps that appear to offer prebooking do operate like you described though so I wouldn't be surprised if a degree of that does go on with regular firms too. But if so they should be absolutely clear about that (which I suspect they aren't being).
It will never be easy or quick to move 20K people. It does take time. Which is why an earlier finish would be prudent.
True. But if you are someone who is travelling I don't think it is unreasonable to see there's a train due at 23.50 from a station just half hour walk away and plan to get that.
I do agree the organisers should have done better though, not just with the timings but with the general logistics (it did feel a little chaotic there in some regards lets say - again probably because they haven't actually held many similar events before). Though I also feel the railways should have planned better too. They were able to magic up an additional service in the end so why wasn't that just planned for beforehand anyway? As someone else has mentioned it does look like GWR have put plans in place for some things in Cardiff on the 28th, so why couldn't the ToC here have done the same?
Missing the end of a concert to get home by train is nothing new, I still remember missing all the Genesis encores at their Abacab concert at the NEC December 1981......
To be fair, there is missing the end of the concert and missing so much you may aswell not have gone. You say about missing the encore - if someone had left before the encore they may have managed to get the 23.03 if they were lucky, but probably would have actually just ended up waiting around for 50 minutes at the station and getting on the same 23.50 service they would have been aiming for had they stayed till the very end of the concert.