Unidentified silver proof coins?

i have set of 12 silver proof coins depicting various scenes of famous london historic landmarks under construction eg, big ben, the london bridge, nelsons column etc etc on the reverse of all these coins is the british museum 1998. they are hallmarked on the outer edge london 1997/8 with makers mark initials PM which i think is pobjoy mint but i cant find these coins anywhere? does anybody know about these coins they are twelve in the set each 30mm diameter and weigh 10.2g.