Hi. it depends on the time of day. Personally wouldn't use A312. I would head for the M4 spur which goes straight into LHR. Turn left just before the tunnel (stay in the left hand lane). The turning is just after the last bridge before the tunnel. Up to the roundabout and take the 3rd exit which aims for the airport, stay left and turn left. head straight down the perimeter road til you get to the end of the runway, round a right hand bend and turn left into the car park. Make sure your early the buses run every 10 minutes on the hr ten past, twenty past etc...... Then every 15 minutes after 1000 til 1800. Give yourself about 15 min at least to get through LHR from 0600 - 0800.
Hope that helps.
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