UK census: In mid-2006 the resident population of the UK was 60,587,000 (...) The UK has a growing population. It grew by 349,000 people in the year to mid-2006 (0.6 per cent). The UK population has increased by 8 per cent since 1971, from 55,928,000.

French Census: Au 1er janvier 2007 (...) la population de la France est estimée à 63,392 millions de personnes dont 61,538 millions en France métropolitaine.

So less than 1m difference, and possibly less if you take into account all the UK expats throughout the commonwealth, who are often in and out of the country.

For practical purposes, the populations are similar enough for overall economy comparisons to make sense. In any case, it is the relative trends that matter (and these cannot yet be ascribed to the effects of one country being run by a "dynamic" president and the other by a "crypto-socialist" PM...)

In the long run, we're all dead. John Maynard Keynes