No limit officially, down to individual IO these days, hearing lots of stories of people getting warned who are really not doing anything outside the rules, so must be some kind of pattern they are looking for
Its always been possible, the requirements are for proof of onward travel. The airline may ask to see, but in twenty years I've never been asked. Immigration are unlikely to ask unless they are suspicious. Best option is to have a cheap flight or bus ticket out just in case.