A lot will depend on the day you travel and also what time of day you opt to leave. By car at midday during the week I reckon you'd be looking at a journey time of between 3 and 3 and a half hours. It takes me about 2 hours (I live in Acton, W3) but I rarely go over 70 on the motorway, and its pretty much M40 all the way from my gaff. I usually reckon on a good 60-70 minutes from Bromley to W3 but travelling in the rush hour could easily add 30 minutes to your journey time. At the weekend door-to-door in about 2 and a half hours I reckon. The AA (
http://www.theaa.com/travelwatch/planner_main.jsp)
and RAC (
http://rp.rac.co.uk/routeplanner)
websites offer useful tools for working out routes and estimating journey times.