Something like that. 🙂
However it's possible that register.btinternet.com was changed into an IP address on that page due to register.btinternet.com being pointed to a new IP address as a result of the migration plans.
Either way, it won't be long (hopefully!) before that BT Yahoo page no longer exists.
Just to point out this is still happening at 20 March. Contacted BT Help about it, who promptly phoned back to tell me (eventually and after a citrix gta session) (i) It is perfectly normal and the way to deal with it is just continue/ignore (ii) It is not an error but actually a security feature of the website and (iii) - after expressing surprise at the above - yes, it might seem surprising to "an older user". With an attitude to security and customers like this, how can they fail? Awaiting migration with confidence...