The only right of cancellation since May 2005 with TMobile, is the 7 day distance selling regulations, but this is only if the handset is unused or the handset is faulty.
Other networks are going the same way, such as THREE.
They cannot guarantee that the coverage will cover every inch of the UK, and it is not the networks fault if someone previously enjoyed coverage at home and then moved to a home with poor or zero coverage.
TMobile offer Street Check where you can check coverage down to street level.
http://www.t-mobile.co.uk/shop/your-...overage-check/
If you have checked coverage and it turn out that it doesnt work in that street then they give you 14 days to return the handset.