Britain owes much much more than Greece, why is nobody worried about that?
We are very worried about it: have you read the papers lately? Why do you think the current government wants to try to balance the budget?
However, there is no serious risk that Britain will default on the payments due on its debt any time soon. Big G8 economy, decent tax base, etc. Britain has more debt per head of population than Greece does, but is paying the interest and rolling over the bonds on time. Because we aren't in the Euro, we have more levers to pull to control matters, and even though sterling isn't the Euro, it's still an internationally traded, relatively hard currency. We can sell bonds (ie, borrow money) in sterling, and people will take sterling in repayment.
You probably couldn't pay your mortgage off tomorrow morning, but so long as you keep making the payments, that's fine.
Greece isn't able to keep making the required payments, at which point life becomes decidedly more interesting. It is in the Euro so can't just print more money (ie, devalue its currency) and even if it left the Euro, attempts to borrow money in drachma would be met with international laughter. It would therefore need to do scary things involving borrowing money with a promise to repay in dollars or Euros, which involves getting hold of dollars or Euros at the right time. It has very bad choices to make: small economy, weak tax base, illiquid currency if it went out alone.
Britain's the second largest economy in Europe and the fifth in the world (behind the US, China, Japan and Germany, with our overall performance a lot healthier than Japan's). Greece, not so much.