The Tesco home page is www.tesco.com.
You might have expected it to be www.tesco.co.uk. To test this, type www.tesco.co.uk into the URL field of your browser you will see that you are redirected to www.tesco.com. If you look at the home pages of Tesco's direct competition in the UK you will see that this is unusual. Type in www.sainsburys.com and you get redirected to www.sainsburys.co.uk, and ASDA has a similar UK based home page.
As Tesco is a UK based company it seems a bit strange that it would not have a .co.uk homepage. The whole trend for search engines like Google is to be sensitive to local search. So if you are in the UK and search for, say, "online purchase of potatoes" you are more likely to get a UK based site. So Tesco's home page seems, at first sight, to be self defeating.
One possible explanation for the greater weight given to www.tesco.com compared to www.tesco.co.uk is Tesco's recent & rapid expansion into the world-wide marketplace. Maybe they feel that they already have sufficient brand recognition in the UK and that it is better to have a "world-oriented" www.tesco.com site than a local tesco.co.uk site
Sainsbury's have shown little ambition for making its presence felt outside the UK. ASDA is owned by Walmart and has no incentive to get brand recognition outside the UK. It would be rather silly for Walmart to be competing for market share with ASDA. It is interesting that Marks and Spencer also have a .com site. Perhaps this reflects their rapid expansion into the middle east.