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Sunday evenings on the M25 shouldn't be too bad, and the NEX drivers know how to avoid problems if their colleagues report traffic jams. The trains may be subject to engineering problems, and as you suggest, by flying you protect the connection. I'm not sure where your final destination would be, but there may be a LGW alternative.
As for general guidance on this connection, there is a very detailed guide to transferring between LGW and LHR in the Forum Dashboard, admittedly referring a lot to LGW North rather than South, but that doesn't make a huge difference.