Vodafone doesn't have an EDGE network in the UK.
There is GPRS and 3G including HSDPA with support of up to 7.2Mbps and HSUPA.
On older firmware's before 2.0, unlocked iPhone's on Vodafone UK used to have the Edge network logo on the screen but this was just because a GPRS connection was available.
Now on firmware 2.1, unlocked iPhone's on Vodafone in the UK will only display the "o" symbol for GPRS and not the "E" symbol for EDGE.