The main shopping area in the city is the prestigious area around Vaclavske Namisti and there are several western chains represented with large stores, however, the prices in these stores are probably as high, or higher than in the UK. On the whole the best bargains are to be found in the smaller Czech shops, where you will find well-priced antiques, and plenty of Crystal glassware of good quality.
There are plenty of Christmas markets in the city during December, and these are great for picking up festive decorations and presents. You should however be wary of pickpockets and bag snatchers during that period, and not carry more cash than you need to.