A scam webpage about BT 75 anniversary and a prize draw keeps appearing on my website and causes the page I was on to crash
As you have pointed out it it is a scam page it has nothing to do with BT.
You should run the free ADWCleaner to see if you have any unwanted add ons in your browser and on your computer and you should clear the browser's history and cookies.
I had the same scam this morning.I was on the PlanetF1 site and when I clicked a link to a story I had the popup window appear.
I closed it straightaway.