Don't commute. You may even want to try to walk to work if you can - it is easier than it was as more offices become apartments.
If you do commute, stick with the tube and avoid overground National Rail. Whatever you do, try to avoid having to change trains. If you assume that 10% of underground lines have a major failure every morning, you are doubling or tripling your problems if you use 2 or 3 lines to complete your journey.
A monthly rent bill of £2,000 would get you something decent (assuming you want your own place) - you'd get a decent one-bed in a 'nice' central area for that.
Ignore everything else you read about London being 'expensive'. It doesn't matter if Starbucks is 10% more expensive here than Chicago or that a luxury restaurant meal costs 15% more. That isn't an issue on £70k. These sums are all totally irrelevant compared to your rental bill - focus on that and then don't fuss ever again over the price of anything else in London.