Hmm.
The nearest point to park for Mile End, coming from the A2??? Possibly Dover?!!
I still can't think of anywhere particularly obvious, or find many possibilities listed online. If you could find a space, there's an incredibly cheap (council run) car park very close to Plaistow station:
http://www.carparks4u.com/PLAISTOW/SETTLE%20PO INT.htm
From there, it's just 4 stops on a westbound tube (District Line or Hammersmith & City Line) to Mile End (or 1 stop further to Stepney Green).
However, my big concern would be that there are only 68 spaces. On a weekday the car park will probably fill up with season ticket holders very early on.
My own instinct would be to avoid the hassle of the Blackwall Tunnel and the incredibly poor signposting around that part of London. (Satnav doesn't help much when you get stuck in the wrong lane and nobody will let you move over!). I'd simply turn right on reaching the M25 and go through the Dartford Tunnel. At Junction 29, I'd head in towards central London on the A127. A short distance along, you pick up the signs, telling you to turn off to the left, to Upminster. Just follow those signs to the station. Go past the front of the station and turn immediately left. That takes you to Upminster station's large (NCP run) car park, with 540 spaces. (�6 per day. The car park gets fairly full but I've always managed to find a space). Then it's 15 stops on the District Line to Mile End.
Chris