Shame that a 12 inch Powerbook is out of the question as this is close to ideal (lovely thing with 6-pin firewire and USB 2 and apart from the need for an airport card, could almost have been made for it). Also, the Mac version of PSCS needs no activation nonsense.
Anyway, that aside, the PC market is absolutely chocker with choices. You need to think carefully about what exactly you need it to do, i.e. what ports, are the main considerations processing power, quality of the display or portability? Inevitably, there will be trade-off between these considerations. If ultimate portability is the thing, you would do well to look at the JVC mininote series which are about the size of a hardback novel, with a small, but very high quality display. You will need to buy the external DVD writer and more memory to go with a machine like this, but they are very popular amongst pros and Calumet sells them (e.g. in Bristol). Apart from that, I suggest you look at some mags e.g. the latest PC World Magazine has a review of desktop replacement machines, which should be plenty powerful enough. It also has some longer term recommendations such as a subnote laptop favourite.
I do not really understand the need for a physical outlet. In the UK this will mean the dreadful Dixons or the equally dire PC World (the shop). The burger flippers who work in these outlets will not know their a**e from their elbow when it comes to advising you on what is best. Personally, I always buy from ebuyer.com as they are the cheapest and they have a very good returns policy.