The railway line above you at this location is between London Waterloo and Portsmouth Harbour.
Find the plaque confirming this is the Forest Road bridge
This is bridge <number1> / <number2> WPH1
Take number1 away from number2 to give you the ANSWER.
The cache is at N51° 02.(876+ANSWER)' W0° 52.(076-ANSWER)'
The cache is in a M&S Mint tin; please place the cache back in the same location