It should just be ibank.barlays.co.uk (no www). That should sort out the certificate error.
The reason it appears is because the two versions of the web address are not identical. The computer does a comparison with the web address you type in and the address on the security certificate, and because they don't match exactly, presents an error.
In this case, there is no security risk because you can be sure that both versions of the web address point to the same site run by Barclays, but normally, you should always refuse to continue if you have a certificate error - you can't be too careful!