Yes, three years ago we did the complete circuit from Brentford into Paddington on the GU and through London on the Regent's Canal to Limehouse and back up the Thames from there. No problems for an 11ft wide 60ft widebeam - apart from the weed and rubbish in places and a few dry pounds! But that's fairly normal in most urban locations we have found.
Moorings were quite hit and miss, this was in August, but I gather that there has been some proactive work since our visit to free up some of the visitor moorings from long term live-aboard 'squatters'.
Well worth the trip. We spent a month pottering about - it was the summer of the riots, and every time we moved to a new location it seemed there was a riot there! But we came through unscathed!