This is most likely because you are not based in the UK and the content cannot be accessed outside the UK. This only applies to a small number of video links. You may find that it's working OK at home where you might be using a VPN to access UK content and then discover it doesn't work within your school. It is best to be prepared in advance of these situations so that you don't discover the problem in the middle of a lesson. The easiest solution would be to hide the slide where the embedded video or video link does not work. There are other solutions, but please do check the legality of these.