I think the answer is "you can't".
I assume this will be the number you are trying
Google UK Ltd
76 Buckingham Palace Road
London SW1W 9TQ
Phone: +44 (0)20-7031-3000 Fax: +44 (0)20-7031-3001
I would suggest phoning the number, and then when asked for an extension dial a 4 figure number (starting with 3) such as 7031-3123. After a bit of trial and error you might reach somebody on an extension who could direct to an extension for Maps. Who knows, you might even dial direct to the Managing Director!
It must be worth a try - good luck