Upvote:0
UK Tourist Visas are a maximum of 6 months stay within the country. You will have to exit the country and apply for a new visa or if you visa hasn't expired then you can re-enter the country. Although they will be pretty suspicious if you stay for the full 6 months then re-enter.
Upvote:2
No, you may not extend your tourist Visa for any period longer than 6 months. The maximum allowed is 6 months. If your original Visa was shorter than 6 months, then you could apply for an extension up to 6 months from your date of arrival. But in your case, your six months are almost over and your Visa was for 6 months.
See here for more information.