You'll have no problem buying cigarettes in Prague but you should remember that although the Czech Republic is now in the EU, there are still tight restrictions on importing tobacco products into the UK.
From the 'old' EU countries (e.g. France, Spain), you're allowed to bring back a (theoretically) unlimited quantity of cigarettes for your own use but HMRC quote a maximum guideline figure of 3200 cigarettes. (Above this quantity, Customs officers might start to ask questions).
From the 'new' EU countries (including the Czech Republic), you may only bring in 200 cigarettes. If you bring in more than this, you must pass through the red channel and pay duty:
http://customs.hmrc.gov.uk/channelsPortalWebApp/channels PortalWebApp.portal?_nfpb=true&_pageLabel=pageTravel _ShowContent&id=HMCE_PROD_009560&propertyType=do cument
Chris