I can already use it for free on my Smartphone and PDA why not an iPhone?
Cos they know you're probably paying £35 a month for your phone calls, so you can afford it.
Seriously though, it's a new and quite significant feature, that wasn't available when you bought the software, so it doesn't seem that unfair to charge extra? Presumably it cost them something to develop (iPhone development is annoyingly expensive for a small company too, especially a company that doesn't use Apple stuff or develop for Mac already).
£20 doesn't sound like that bad value if it is a usable navigation system – if you wanted to buy a Garmin or Satmap standlone GPS with OS mapping, you'd be paying hundreds of pounds.
Joe