It’s a commercially biased link, or “scam”. Only risky if you follow it through.
Got the same screen yesterday when logging in to bt webmail on the iPad. Not on the Windows pc, though.
The iPad seems to be more vulnerable to ads.
I clicked it and dismissed it.
Clean up your cookies afterwards, as it leaves a lot of cookies behind.
Spam, It'll most likely install nasties on your device if you click anywhere on it.
A simple IP lookup will see a BT IP address, which the loads pre-made banners in the form of your ISP to look genuine, simple enough to make.