Really think your question is impossible to get any sensible result from given that you have no idea what the overall income is. Maybe you should ask what percentage in relation to disposable income left after essential bills, mortgage etc perhaps?
I have no mortgage or rental costs so am very lucky in that respect. But reality is I can't afford any on my income and as stuck where I am though if I could afford to move I would be better off as it would reduce my travel to work costs and give me more disposable income.
My childcare costs are about 8% - school hols only (and that includes all my net income, salary, WTC & CTC & child benefit). I do get additional CTC to help towards these costs but it's still 8% of my overall income including this top up.
Council tax is 10% of my income.
Petrol & parking costs for work another 20% and that's not including the car running costs but I own my (ancient) car so have no other costs on it but will have to replace it (with what money?!) soon.
At the end of each month my disposable income after paying for all essential bills, food etc is in the region of £50-£75 if I manage to stick to my budget. And then school is always demanding a big cut out of that for school trips, photos, dinner money etc so in reality I have far less than that....
I do put about £20 a month away for emergencies though, but at the moment am overspent by £200 on my emergency budget so its gonna take me a year to pay that off. Lets hope I have no more emergencies!
I get a little money from my ds father which I use specifically on him though, music and swimming lessons, scout subs and camp and the like. So i'm fortunate to be able to do that.
So no way I could afford a mortgage or rent!
Basically you spend what you can afford to on things. If you're looking to buy in Canary Wharf area then you must have a fairly affluent lifestyle and good income I would assume and therefore your main concern is how much of your disposable income will be lost of you had to rely on 1 income only. There's always things you can do if you have to take a work break. Like getting an interest only mortgage for the time you are at home until you return to work etc..
There are some very expensive but also some much 'cheaper' housing areas not very far from canary wharf/tower bridge areas. So it all depends where you are willing to compromise either short term or long term...