http://www.ukba.homeoffice.gov.uk/vi...ndingyourstay/
If you come to the United Kingdom as a general visitor, you are allowed to remain here for a maximum of six months (or 12 months if you are acompanying an academic visitor). When you enter the United Kingdom, the length of your permission to enter will be stamped in your passport.
If we gave you permission to enter for three months and you would like to extend your stay to the maximum of six (or 12) months in total, you will need to apply for an extension using the application form FLR(O). You can download this form and the guidance from the right side of this page. You should read the guidance notes before you complete the application form.
As long as you tell them when you arrive at passport control then you should be OK.