I remember talking this over with a friend - I have a dd1 born 02/00 and her son was born 05/00. He had to start school in 09/04 - my dd in Scoatland started school a full year later, in 08/05. She was 5y6m and is second oldest in her class.
I did the same with ds, also a Feb birthday. Dd2 is Nove,ber and I felt awful sending her to school at "only 4y9m - I really felt she was too small and she has certainly not flourished at school the way the other two have.
I think some of the potential problems arise at secondary school - I taught in secondary before I had the children and you could always spot the boys who were 11 and a half rather than 12 and a half - not just size, but they were almost always the ones with no homework/forgotten folder etc....
In Scotland though there is a year less at secondary school (only 6 years) (and consequently most degrees are 4 rather than 3 years) and one of my big issues was I didn't want my dcs leaving school at 17.6 like I did and then have trouble getting a Gap year thing to do (many places look for a minimum age of 18). For my first term at Uni my student card was a different colour to other people's and had "MINOR" stamped across it so I couldn't get into the Union after 8pm!
But I look at my flourishing kids in p5(y4) - just turned 10, p3(y2) - just turned 8, and p2(y1 - was 6 last Nov) and I know I made the right decisions for them. Nowhere in Europe makes children start school as young as England.
Actually, last year we did consider moving south for a bit - there was a project dh was interested in working on and as I am a distance learning student it was no bother for me. I phoned the LEA where we may have gone to to ask about school and was told there would be no option but to put dd1 and ds "up" a year, as that was the law! So they would happily skip an entire year of education for a child from another country just to have them in the right year. It put us right off moving, and is still one of the major reasons I would not move to England.