There is watch related crime in London, and it has increased in recent years, particularly moped based snatch and grabs.
But by the same token, there are hundreds of people in the city wearing Rolex AP RM PP et al on a daily basis and the MET sting operations are widely known about.
It also depends on WHAT Rolex you're wearing. The more sought after the model (sports/professional) then the higher the likelihood of them being on the steal to order lists.
There are some which fly more under the radar like Exp / Exp2, OP, five digit models and not the honking great big ceramics, bla bla bla.
Also depends if it's going to be visible with a t shirt or under a cuff.
Lots of factors to consider I guess but if your gut is telling you to leave it at home, and if you have a safe place to keep it at home, that's probably where you will be most comfortable leaving it.
Personally, when I'm office based in central London and if i've got my arms out in summer, I wear something less instantly recognisable.
In winter I don't care because everything is covered by sleeves.
Edit - May be worth noting also that women's rolexes are less targeted for theft, so there's that too.