I can't work out why you're looking for somewhere north of the Thames.
I know that places like Romford and Basildon are far more affordable than Wimbledon itself (or, even more expensive, areas like Richmond) but I would have thought that Croydon might make a lot more sense. Any extra costs of housing would probably be offset by the far lower commuting costs. (Wimbledon is directly accessible from Croydon, on Tramlink).
Driving from Romford or Basildon to Wimbledon either means paying �8 per day congestion charge (and suffering horrendous traffic in central London) or going right round the M25 and coming back in on the A3. The train journey takes about an hour and a half and would involve travelling on the Tube at peak times (which is certainly no fun during a heat wave). Tramlink from Croydon takes about 20 minutes (which would give you about 2� hours extra leisure time every day).
For other possible alternatives, south of the Thames, check out areas such as Clapham, or possibly Balham.
Chris