onward ticket is NOT required for visa exemption. Or tourist visa from London Embassy. But the airline may ask for return/ onward ticket. And then can deny boarding if they want.
it depends. I got a tourist 60 day visa with return flight 189 days later. Usually, they only want to see flight departure from country you applied for visa from.
the airline are more likely to ask, not immigration. I've never once been asked by immigration in over 30 arrivals. But the airline often ask. The local embassy know nothing about immigration entry criteria.