Unfortunately, at £100 per weight (and £1,000 for the whole set!), it is a bit on the pricy side; if that's out of your price range, you may wish to consider just using Helvetica and hoping that nobody notices. (And, to be honest, few people would be able to tell the difference.)
(Note that if you decide to be even more thrifty and use Arial, people will laugh at you.)
And here is a detailed article on the evolution of the London Underground typeface, from Edward Johnston's 1920s original (which influenced the design of Gill Sans), to its subtle redesign by Japanese typographer Elichi Kono in the 1970s (Kono's account appears here), and other adaptations made recently as it was adopted across the entire London transport system.
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