[bok]

A Brit can come and go as he pleases (no visa required) unless, he establishes a residence, etc....

They can do this for eleven years?!? Is this an agreement reciprocated between the US AND UK?

[An.American.Expatriate]

More or less, yes. There was a Visa Waiver Program for several countries (mostly Europe). This program officially expired, but is continued anyway.

Under the rules, a Brit could enter for a three month period without a visa. It is non-extendable. If the Brit were to leave the country, he could return for another three months, etc...

Theoretically this could go on forever. The only restriction is no residence / employment.

The agreement is/was reciprocal (you don't need a visa to travel to Europe, unless you want to work).