Take the tube (Piccadilly Line). What's the use of taking the HEX if it will only drop you off at Paddington (then you would need a bus or taxi to take you somewhere interesting). Mind you, if you are arriving on a Sunday, things are pretty quiet in London until around 11 a.m. /12 p.m.

Piccadilly Circus is a good place to start. You could head on up to Leicester Square and from there to Trafalgar Square (from there a few options like Buckingham Palace or Houses of Parliament/Big Ben), then depending on where you end up, head towards Oxford and Regeant Streets (mostly shopping and coffee shops) or do Hyde Park then take the tube again from Hyde Park Corner (Piccadilly Line) back up to the airport. Just a suggestion (especially if the weather is good and you don't mind a good walk).