From UK Border Agency :
If you are coming to the UK for a short visit (up to 6 months), you
generally do not need a visa.
However, you should bring documents with you to show our immigration
officers when you arrive at the UK border.
The Visiting the UK section describes the different categories of
visitor visa, and lists the types of document that you may want to
provide. It also explains the conditions of your stay as a visitor.
Although you do not need a visa, you may want to obtain one before you
travel (for example, if you have a criminal record or you have
previously been refused permission to enter the UK). And you will need
a visa if you want to come here in some categories of visitor (such as
a visitor for marriage or civil partnership). The Visiting the UK
section contains more information.
If you travel to the UK without a visa, you should bring them with you
so that you can show them to our officers at the border.
You should provide as many relevant documents as you can to show that
you qualify for entry to the UK. If you do not provide them, we may
refuse your application.
You must decide which documents will best support your application. We
advise you to consider providing documents that contain:
- information about you
- information about your finances and employment
- your accommodation and travel details
- information about your visit to the UK