I too am in the UK, have the correct time & date on the computer clock, can access https://api.posersoftware.com without issue, have disabled my antivirus while installing, and have the same issue when installing on different computers.

in case it helps, when trying to start Poser with Bitdefender enabled, the log shows:

The application Poser.exe attempted to connect to the Internet: File path: D:Program FilesPoser SoftwarePoser 11Poser.exe Destination: 52.3.70.55 Protocol: TCP (6) Port: HTTPS (6) Bitdefender Firewall has granted access for this application.

(I have spoken to a friend that lives nearby who has not recieved this particular error when upgrading - although they did get an error regarding 'the grace period for activating this product has expired' )