Assuming you meant 'a leave to enter visa' - it's a document issued separate to a persons passport, granting the passport holder permission to legally enter the UK. IT IS NOT, however permission to stay here indefinitely. If you stay beyond the expiry date, your details will be circulated on the Police National Computer as an absconder.
Yes
No need for a visa, as the UK is a participant in the Visa Waiver Program.
NO, in most cases you do not need a visa to come to the UK for a short visit
No UK citizens do not need a visa to go to Singapore just like Singaporeans do not need a visa to enter UK
no
No - you still need a visa to enter the UK !
South Africans require a visa to enter the UK, but it is not a difficult process to get one; just start the process in good time. You may not know that South Africans now require a visa just to transit through a UK airport.
no
Yes
No you may not
No - you can enter the UK for 90 days without a visa for tourist purposes
It depends on what it says on your entry visa. If you enter the UK on a visitor's visa, it usually allows you to stay for six months, after which you have to leave.