If you plan on travelling with bus/coach in England, then the National Express is an easy way to do so. They have great services covering the nation, and if you do your booking early, you can also get very good prices. We have written the different articles on how to get from the different London Airport to London, but in all cases it is also worth checking the National Express, as they have buses travelling from the different airport to the city centre of London, and also from one airport to another.
Website of National Express: National Express
You can also travel with train if you prefer that. If money is important, check out and compare both the prices and travel time.