Several hotels around LHR offer discounted aircrew rates. All subject to availability, but work on around £40 per night for the likes of the Ibis & Premier Inn. Privately owned B&Bs can be considerably cheaper.
I have found, coming from the other end of the country, that commuting costs (around 4 trips per month and the odd night in a hotel) are no more than what I used to spend on fuel, driving round the M25 five times a week.