Do you rent in the private sector? The London Fire Brigade will give you a free smoke alarm after becoming fed up of waiting for landlords to do it.
In October 2015 it became the law for landlords to fit a smoke alarm on each floor of their properties, and carbon monoxide alarms in rooms with solid fuel burners. There's a £5,000 fine if they fail — which is apparently bugger all deterrent, because the Fire Brigade has realised it has a pile of free alarms sitting around that haven't been claimed. So it's opening up the offer to the people who are actually affected: tenants.
There are some strings. You need to apply online and collect from one of only four points around London — and then fit the alarm yourself. You're also not eligible if you live in a 'house of multiple occupation' (i.e., three or more tenants in one household, or sharing toilet, bathroom or kitchen facilities with other tenants).
Which is a shame, because that rules out most houseshares and the dodgiest overcrowded properties.