The ones for £100 are the RCD510 Delphi units. It was made for the Chinese market. The key differences between this and a 'regular' RCD510 are:
Built in USB/iPhone support, meaning no need for an MDI kit
No RDS, so the radio stations have no names
I have one in mine. It's good, but I may be upgrading to an RNS LED soon. I don't listen to the radio so things like DAB and RDS don't concern me.