Option 1 is a good idea, but scrub the taxi, there's about 5 bus routes that will take you the half a mile or so down the road to KXSP, for the princely sum of £2.
Option 2 is probably no cheaper, and a lot of messing about
Option 3 is going to cost you an absolute ton of money, as the GatEx is a premium service, and costs a fortune!
Hope that's helped. I'm a London bus driver, and have lived here all my life, so I have learned from experience!
Barry