@Olestokie 

Hi,

I don't know the exact situation in the UK, but in general this is pretty much the norm in Europe. 
In each country there are like one or two major telecommunications companies that have built the whole hardware infrastructure (i.e. lay the cables all through the country up to every single house and install the necessary supporting hardware along the way) and then there are usually several other providers in each country who just "rent" that infrastructure and resell it to consumers under their own branding.
In order to stipulate competition in the marketplace there are usually laws in place mandating that those who own large parts of the infrastructure must be willing to rent it to other players for a fair price.

And yes, Internet Service Providers generally seem to have the habit of making it as difficult as the law permits for consumers to replace the ISP's "Super" router with another third-party router of the consumer's own choice.