No probs Matt - and thanks John, that's a good point (I'm enjoying a 45 this weekend so wasn't paying attention to planned outages...).
Apart from the extra traffic due to diversions, the route I gave should work up as far as when you come back over Westminster Bridge.
If you're still OK for time you could double back over the bridge for another view of the Eye and thengo along the south bank to Blackfriars, turn left over the bridge, then head into the City. Go under the flyover and turn right into Smithfields for a look at Smithfields Market where you can drive right round the market and then left back onto Blackfrairs Road. Down to Ludgate Circus where you turn left into Ludgate Hill for a good view of St Paul's. From there, start heading north up towards Islington to pick up City Road which leads back to the A40 and on towards Baker Street.
I had similar problems when the 10k was on. Just persuaded my pax to do a walking tour. Dropped then at 1000 at St Paul's and picked them back up in Lower Regents Street at 1630. Nice when it works out like that...