Well, if you counted the number of ships flying the Red Duster, it wouldn't take very long. Counting the ships that are British owned would take somewhat longer, though. If you then extend the count to ships that are insured in Britain, the numbers should start to stack up.
Regardless of the size of the British Merchant Fleet, it is the effect that disruption would have on us that would be the salient point. Disruption to a bulk carrier, say, under any flag would affect us if it was destined for a British port. Additionally, we probably need to view it as the effect it could have on Europe as a whole. A strength in South Africa is a strength that "we" don't have to provide. Just my view, for what it's worth.