It will probably happen. The reason for is simple though. Japanese market has simply shifted to handhelds. Handhelds included this gen have been by far the biggest in Japan so I always laugh when people say that GAMING market has shrinken in Japan this gen. Home console market sure but not the overall market.

UK First-Half 2011
Total SW Unit Sales: 23,326,633 (-12.1%)

Japan First-Half 2011
Total SW Unit Sales: 22,721,979 (-20.8%)

This include PSP, DS, 3DS and PC.

I don't really think that it will, especially not this generation.

If it does, it will be solely on the back of Xbox 360 sales. For the two platforms it actually supports, Japan will remain a much larger market.

probably sometime next gen

Could you possibly be any more vague?

yes! yes i could. i'll look at th op again and give an estimate later

Well I think it will take over by January of 2012. Christmas season will really help the UK take the lead.

As far as i'm concerned the UK is now the second biggest market for home consoles. 

Before the end of the year, largely due to CoD.

I think (and hope) this is gonna take a while. Japan is shrinking but still strong in hardware sales. For example the september numbers:

UK: 158k
Japan: 274k

This comparison is largely academic for two reasons:

1) Based on population, home consoles are far far more popular in the UK than Japan already. Penetration on a household basis for consoles is twice as high given a UK population of about 60m and a Japanese population of 127m

2) The UK supports three competitors, Japan really only supports two - so the potential for the Japanese market is actually higher for home consoles than the UK which seems kind of close to maxing out honestly since consoles are already twice as popular on a household basis as in Japan.