I am doing the same as you running Brighton and then London, my only plan is to start training earlier than my usual 8 week marathon plan i.e. now. Have a good build up and then race Brighton and enjoy London.
My reasoning for racing Brighton not London is that I can imagine with all the crowds at London it will be a nightmare running through the crowds so I may as well enjoy it.
I have a 2 year old and 10 month old, running takes a back seat I'm afraid but I find as an early riser I can do a long run saturday morning and be back in time to not miss too much.
Then club night is allowable, anything other than those 2 runs is a bonus but as they get older it is easier, the first few months with both were a right off though.
I have showers at work now though so I can run 6 days a week (5 worl days plus once at the weekend) if I like, most of those being at lunch time. Before I had the showers available though it was pot luck. Depends on how understanding your wife is and how much you want to do.