If someone 'cuts you up' and then immediately 'slams the anchors on' as they do so, you've got an excuse for running into the back of another vehicle. The police have collected statements from other drivers specifically to eliminate such a possibility. .
Unless another driver takes actions like the one's described above, you will normally be deemed to responsible for a rear-end shunt. At typical motorway speeds, the minimum stopping distance on a dry road, with an alert driver, is around 24 car lengths:
http://www.direct.gov.uk/prod_consum_dg/idcplg ?IdcService=GET_FILE&dID=95511&Rendition=Web
(Those who teach advanced motoring skills recommend keeping at least 30 car lengths between vehicles at 70mph).
In rain, you need to double those figures.
If you're driving at 70mph, you have a duty to actively seek to maintain those distances from the vehicle ahead of you. (i.e. if someone pulls into the space between you and the leading vehicle, you should lift your foot so that you can drop back to a suitable distance). You should also remain alert, so that you can respond immediately if the brake lights show on the car ahead of you.
If you're too close to the vehicle in front, and/or you fail to react in time to its brake lights, you're guilty of driving without due care and attention. It wouldn't matter if the other driver had drunk a bottle of whisky and performed an emergency stop just for the fun of it. The drunk driver would be prosecuted but you'd still be guilty of an offence as well.
Chris