My latest best guess on this is a Canberra, but I feel the need to explain myself.
Does it look like a spitfire? no, but knowing how vague a non-aviation person can be it seems in my experience that to them everything in the sky
either looks like a spitfire or Concorde, also the large B-type roundels under the wings (ie the dark blue and red ones with no white in them) can
easily be mistaken for black spheres or discs from below, especially if the viewer doesn't know what markings they are looking for. The fact that
there are no silver canberra's however means that my theory is not looking too good, unless I can call in the old 'trick of the light/glare of the
sun' cop out