The networks have online coverage checkers on their sites, and should coverage adversely affect your experience you have statutory 14 days to retun your phone for a full refund
Apart from T-mobile, they do not offer a 14 day return on retail contracts. T-mobile perform a coverage check on your post code before connecting and inform you of any poor coverage in the specified area. If the actual coverage you experiance conflicts with the online coverage map they provide then a full mpv check is carried out and then most of the time an investigation ensues involving many phone calls and some letter writing inorder to get the contract cancelled.