jabird - as long as BAA do indeed sell one of either Edinburgh or Glasgow, I'm not sure why this would create competition issues.
Assuming either EDI / GLA were sold to a company that doesn't own either Dundee or Prestwick, then you would end up with up to 4 potentially competing airports in the central belt. Yes, I am stretching this a bit, but people in Falkirk can credibly choose either GLA or EDI, those living in northern Edinburgh can credibly choose DND and those living south Glasgow can credibly choose PIK
The only problem with this, is that there probably isn't enough room in the market for 4 separately owned airports to compete for the traffic. While London shuttle services will be unaffected, I'd expect EDI and GLA in future to compete very fiercely with each other, particularly for the sunshine routes and charters.
If anything, persuading FR to move from PIK to GLA is in BAA's interests, as any buyer of GLA would see it as a signal that PIK is in very deep trouble, and could be expected to lose further passenger traffic, effectively removing one competitor in the central belt, making GLA more secure and profitable, and thus drive up the price that buyers would pay to BAA to own GLA