It's a very difficult thing to put a number on. Firstly France does not collect racial stats by law, so any estimate really amounts to guess work, and on top of that, France considers it's Overseas Departments to be integrated parts of France, unlike the UK where its overseas territories are considered separate to the UK.
Estimates for France vary widely from 1.5 million to 4-5 million. I'm not quite sure how you can get an estimate which varies so widely, so I'm assuming that 1.5-2 million means Metropolitan France, and 4-5 million is including France's overseas departments.
As of a 2007, it is estimated there are 1.5 million people of full Sub-Saharan African heritage in the UK. This count does not include half-blacks, etc. If you include half blacks in 2010 you're probably pushing 2 million.
All in all I would be inclinded to say France has more, but as I don't know whether their estimates just account for pure SSA, or mixed I don't know, and also I'm unsure whether the estimates are just Metropolitan France, or include the Departments.
Hopefully someone can shed a bit more light on this?