This is a hard one. Also in London, I get the feeling that most of the Chinese speakers around you are Cantonese speakers (Gerard street, etc.). I think it depends on what type of learner you are (visual, audial (?), etc.). I think is also depends on your level and what you're focused on (how is your reading? Do you want to focus on that?) Personally, I've mostly given up writing characters. Chatting with Chinese people online (IM) is good way to keep your basic character recognition skills up (you only need to remember pinyin in those cases). Your Chinese partner will inevitably be using many characters you don't know -- so have a dictionary program handy (I use wenlin). I think this is also helpful for building really useful vocabulary (your language partner obviously uses it). I think I am more of a audio learner generally -- so I'd keep listening to podcasts (chinesepod, for example). Those podcasts are free, and it's good to keep hearing Chinese if you don't have the opportunity to speak it that often.

Why not learn from your coworkers? I think London is a great city with diversity.  There must be a lot of people from different countries.