Five more points for Ulster as they strengthened their position at the top of the British and Irish Cup Section 1 but the quarter-final place will not be decided until next Saturday when they travel to Jersey.
The visitors at Kingspan deservedly led 14-12 at the interval but Ulster, packed with star players for this vital game, dominated the second period. Their backs, with Craig Gilroy and Darren Cave much in evidence, exploited gaps in the visitors' defence to run in four further tries.
After six minutes Peter Lydon converted a George Turner try for the London team but Ulster stormed back with Johnny McPhillips and Lorcan Dow scores in the second quarter, the latter goaled by McPhillips. A Don Koroi try with the help of an off-the-post conversion gave the visitors the interval lead.
Ulster were soon in control and a pile of bodies over the line appeared to give the try to Robbie Diack before Cave ran in two more, the second after excellent work by Gilroy.
Both were converted but the Scots fought back with converted Joe Atkinson try. Finally, winger Dave Busby crossed for Mark Best to add the points and although the visitors had the better of the play in the final ten minutes and were held up a couple of times over the line, the home defence held.
ULSTER 'A' - R Lyttle; D Busby, D Cave (c), M Best, C Gilroy; J McPhillips, J Stewart; T O'Hagan, J Murphy, R Lutton, R Diack, J Donnan, S Mulholland, C Joyce, L Dow. Reps: Z McCall, C Trenier, A Thompson, C Smyth, J Creighton, R Butler, J Owens.