All UK tourist visas are issued for six months from date of entry. Visas issued prior to travel are stamped with a valid entry and exit date; these visa holders can't enter the UK prior to the valid entry date and must leave by the exit date--even if they arrived later than the valid entry date. Those who secure their visas upon arrival, however, have no "valid" entry date; their date of entry is simply when they choose to arrive; they receive the six-month visa stamp while going through immigration. No UK tourist visa can be extended beyond the six-month-per-visit time limit. It's illegal to stay beyond a visa's valid date, and doing so can result in being banned from the country for a decade.
The UK does issue a 10-year multi-visit tourist visa. Holders of this special tourist visa can enter the UK as many times as desired in a 10-year period, never staying for longer than six months at a time. Pre-issued tourist visas will show the number of times the visa can be used to gain entry to the UK. Standard visas state "01" under entry terms; tourists allowed to enter the UK twice, for two separate six-month periods of time, have visas that read "02". Those with a 10-year multi-visit visa will see the word "MULTI" on their visa. Tourists who receive their visa as a passport stamp upon arrival aren't eligible for multi-visit visas; all travelers who want special-term visas must apply for them prior to travel.
All tourist visas fall under the same conditions. Tourists cannot work while in the UK and don't have access to public funds or public health care, outside of an emergency. Tourists are expected to carry their own health insurance and must be able to prove they have enough funds to support themselves for as long as a six months. While travelers who have to apply for a tourist visa ahead of time must provide such proof to the British government to secure the visa, those who receive their visa as a passport stamp at the border may or may not be asked such questions by the immigration officer.
No one entering the UK, whether citizen or foreign tourist, may bring in meat or meat products, dairy products, fish, honey and certain fruits and vegetables--regardless of the reason. In addition, no one entering the UK may bring in weapons--including firearms, explosives, drugs, obscene materials or counterfeit goods. Rough diamonds, endangered plants and animals or products made from endangered plants or animals are all banned unless the traveler has a permit to carry such items.
