If you have a compass to hand place the watch next to it.
If the neadle moves away from north it's likely the watch has been magnetised. (An iPhone compass also works)
If it has, it's a straight forward process to solve.
I bought a cheap demagnetiser from amazon. They work a treat if you fancy doing it yourself.
If not I am sure the watchmaker in Piccadilly Circus station could sort it out. (Can't remember his name but he is in the ticket hall).
What part of London are you in? If west an alternative is to try the guys next to Goldhawk Rd Station, cheap and cheerful but will get it sorted for you.