Many had failed, even I when I did my domainseek.co.uk, as all instructions you find doesn't work.
Take a look at This WineStar.co.uk and www.winestar.co.uk
Now Here's How I did it:
1) Add a CNAME record on your 123-reg DNS, it should be www and Type : CNAME and the content should be ghs.google.com. <---- don't forget the full stop in the end or Blogger won't find your blog.
2) Then "important" delete the original www record, now your DNS setup page should look like this.