Hi Katie,
I'm out to my wife and kids and their partners, plus my wife's younger sister...and therefore most probably, the rest of the English speaking world.It can be risky coming out to one person at work if you want to keep this secret, as it is no longer a secret if more than just you knows.
Alternatively, you could look to a local group to attend or contact. That way you maintain your privacy, without the risk of being outed amongst your fellow workers. Now, having casually dropped that in, whereabouts in London are you? PM with that info if you want. We have a group in Croydon that meets in south London every month if you want info. One coming up very very soon, so book early!
Becky