When you say "Voyage Business Unlimited for £20", I presume you mean Voyage Business Unlimited for £20?
If it is then beware, Vonage tend to lock you into contracts, charge you early termination charges, disconnection fees, lock down their hardware and the price you see is usually not the price you pay. Notice how you can't order online? You have to "call now to order or request a call back". I recommend in this case that you best request a call back and discuss your requirements with them directly. If you do, please come back and let us all know what they said and how much they quoted you.
In the meantime, any reason why you want to setup your own asterisk PBX server and not use a 'hosted' or 'Virtual' PBX? From what you wrote above, it appears that you require six extensions? You would only be saving yourself around £6 a month. If Vonage offer the ability to register and use six phones on their unlimited offering at no extra cost then i'd recommend that you use their PBX. Have a read of this: http://www.voipfone.co.uk/premises_vs_hosted_pbx.php
If it helps, a hosted PBX consisting of six extensions and one UK geographical telephone number (DID) would cost around £8 a month. However, I must point out that unlimited calls to landlines and mobile are not included.
Finally, this may be of help: WelshPaul @ Confused which way to go - Asterisk or Horizon