WAAS is to the USA what EGNOS is for Europe. e.g. it is REQUIRED for precision approaches (GPS equiv of ILS). The WAAS and EGNOS standards are supposed to be the same and compatible. It increases the overal precision to about 2m. They are supported by NATS so technically when EGNOS is certified for use in precision approaches during 2008 I dont see why the CAA wont add the procedures here. See the Limoges test info.
You are correct it is not necassary for enroute.
It would be cheaper in the near term to already have a WAAS approved system. BUT I suspect that you may also require EGNOS certification (although this should not be required by the standards i.e. WAAS=EGNOS). Given the way that certification takes place I would not be surprised if some form of EASA/EGNOS type certificate was going to be required, so you may have to upgrade later anyway.
Given the current process to adopt non-precision approaches at existing instrument runways, I dont see why the CAA would not do the same for EGNOS precision approaches. I also agree that other European countries (e.g. France) will probably adopt first.
I agree but hope that it will be less that 10 years before we see complete GPS only approaches in the UK, but once establish as a principle on exisiting sites then it is hard to justify not allowing them on new sites, without the need for expensive VOR/DME/ILS ground equipment.