In England, Wales and Scotland, you apply to your local council. The UK government website has a local authority postcode checkerhere, which directs you to the council running services in your area. There are similar tools on theScottishandWelshgovernment websites. In Northern Ireland, you can...
The UK's telecommunications regulator Ofcom has a mobile and broadband checker for indoor and outdoor coverage - you can use ithere. "Although this is meant to be a guide, it can also be relied on as evidence that you are not getting the service you have paid for...