Tha black print on metal is the BT claim that that phone box is the centre of Britain.
The blue Haltwhistle postcard was put there by another as Haltwhistle claims to be the centre of Britain.
I think it all depends on where you measure from-to and what part(s) of "Britain" you include or not.
btw, the Centre of Britain Hotel in Haltwhistle is excellent. We had a wonderful weekend there with David & Shirley.