I cycle from HOP to Old Street everyday all year round and have done for years. It takes a lot to get me on an overcrowded or delayed train! Anyway, echoing some of the above comments, do get cycle training as it makes the world of difference in how you cycle and view cycling (you can get free training from Lewisham or from the borough in which you work).
And if anyone look like me to show them a safe route to the City from HOP, then I'd be happy to show you (just message me).