I do not know the "exact" reason but I think I have found what I believe to be the cause.
Based on what I have experimented with, if you open multiple tabs that send multiple requests to the website, I think it puts a temporary block on your IP.
I have an extension for Google Chrome called "Tabs Back Up and Restore", it allows you to just save your Tabs for you to open at a later date instead of having to select every single tab.
When I opened the tabs that I had saved the day prior, it was about 10+ tabs, the Website must have an automatic request limit for the amount of requests one can do in a certain period of time, and because my IP was trying to request just over 10 tabs in a matter of seconds, the site puts a soft block on the IP, this makes sense because as soon as I switched to another Region in England that was in the same area as me, not even another country, like America or any other place really, it would immediately start working again.
So I would bet money that is what is going on.
I don't know the full technicals with that, but that is the only thing that I have been able to do and it has replicated the same issue every time.
If a Developer of the site can confirm this, it would go a long way to solve this kind problem with people having the same question if this is the cause.