7:30am is too early for the "heat of the rush hour" at Euston. It'll be busy, but it's far busier at around 8-8:15. And the M25 is *notorious* in the morning; it just needs someone to sneeze and it backs up.

I would not even consider road transport of any kind to London (defined as anything inside or on the M25) unless I was aiming to arrive at my final destination at or before about 6-7am. The trains will be busy, but at least they are in general punctual and reliable. (Yes, from experience - the WCML commuters generally get a very good deal).

Neil