After thorough calculation, I think I will use the bike despite the all-bus route (245, then A6, then 42A or 133 or X40) is much faster. The reason is because the National Express A6 + 2 bus single fares from Stansted Airport is too expensive compared to a railcard-discounted super off-peak return train ticket, that I can save £16.33 by taking my bike there with the whole return journey taking 72 minutes longer on the clock (130 minutes longer apparently as I value my riding time to be 150% in-vehicle time, and also another 10 minutes for preparing and bringing my bike down and up my flat).
The best method without using the bike is to walk to Willesden Green to take the Jubilee, then Victoria, Stansted Express and the bus, which is 36 minutes slower than the all-bus route (i.e. 36 minutes faster than using a bike) but saves £9.60 compared to it, but that 36 minutes + riding effort saved still don't worth the £6.73 price difference to me.