If you have mobile phone insurance then you are going to need to understand that unauthorised signal boosters will not only invalidate you warranty if you are found out but more likely than not your mobile phone will not be covered by your insurance.
Not withstanding the fact that mobile-phone boosters are illegal in Britain. You dont always get the best signal in the UK however a signal booster or repeater could get you a £5,000 fine and up to a year in prison. There are still a number of these websites around which sell these gadgets from the Far East however they dont boost anything and are illegal as only the service provider can boost or repeat a mobile phone signal. If you have poor reception then call you mobile phone provider or your service provider.
You can also look at mobile phone specs to see if you are able to upgrade to a better phone with a better or stronger signal. Search this website for more details.