It depends where in London you would like to go to. Heathrow airport is roughly an hour from central London by Tube (Subway), add about 15 minutes if you're landing at Terminal 4 or 5, they are a...
There isn't a direct one. As you are travelling between airports, I assume you've got baggage and will want to avoid long esculators and stairs. I recommend you take the London Underground Piccadilly...