The usual time frame offered for business or holiday travel is for no more then 90 days. As far as I can tell this is not true- It is up to 6 months. But the best way to check for the most recent info would be:
http://www.ukba.homeoffice.gov.uk/visitingtheuk/visitors/eligibility/
Citizens can legally stay forever.
90 days
You can stay in Panama for up to 180 days without a visa.
As a tourist, you can stay in Japan without a visa for up to 90 days.
As a tourist, you can stay in Japan without a visa for up to 90 days.
You can stay in Peru without a visa for up to 183 days as a tourist.
Jamaicans can stay in Singapore for up to 30 days without a visa.
Citizens of most countries can stay in Panama for up to 180 days without a visa.
They couldn't legally cross into the UK without the correct visa. If you're talking about an illegal immigrant - they will be deported as soon as the authorities make the arrangements to send them back where they came from !
The length of time you can legally stay in another country depends on the specific visa regulations of that country. It is important to check the visa requirements and restrictions before traveling to ensure compliance with the law.
As a US citizen, you can stay in Germany for up to 90 days within a 180-day period without a visa.
A Polish national is REQUIRED to have a visa to enter the U.S. So, without a visa, a Polish citizen cant enter the US at all- period (never mind how long he can stay).