Google always redirects you to your country specific Google search engine sites. So if you are in UK, it will redirect you to www.gooogle.co.uk
Now if you want to open google.com follow these simple tricks:
1) Type www.google.com/ncr in your browser address bar and press enter. It will open google.com
2) Type www.google.com in your browser address bar and press enter. It willl open www.google.co.uk. Now look at the bottom-right corner of the page. There you will find a link as "Google.com". Hit the link and Google international english page google.com will be opened.