Belfast could be good. It's got a very interesting central core, a lot of beautiful architecture (the city hall is a gem), good pubs and restaurants, and accommodation is weirdly cheap - I've stayed in guesthouses for around 30-35 pounds/nt with breakfast.

Shopping may be the weak point of the ones you mention - although there's a rather nice shopping mall downtown, and the area of Botanic Street near Queen's University has a lot of interesting bookstores, cafes, and boutique shops to check out.

If you get bored with Belfast, you could split up the week and also go to Dublin, which is only a couple of hours away by bus or train - although it's far more expensive. However, it does have an amazing collection of Georgian buildings - so that would certainly meet your architecture criterion.