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I am flying Virgin in the summer from LHR to SFO and then returning from LAX - LHR. I live in the North West of England, so what is the best way to travel down to Heathrow?

I am thinking of flying as BA and BMI have some fairly cheap fares atm, but is it possible to through check my bags from MAN, even though I am flying with different carriers? Same question for the return journey?

Or, would it be easier to get the train?

Calvin