<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by stut:
If you really want a low-cost option, you can use the Piccadilly Line instead of the Heathrow Express, and just change to the Jubilee Line at Green Park. This interchange is a bit of a schlepp, though.
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There's actually an elevator between the two lines at Green Park, and I think if you have luggage, you can ask a station attendant to let you use it.
Yonatan