I don't agree with that. The UK government will likely only get involved post IAG bankruptcy. At which point it will pick up the remnants of BA at fire sale prices. BA will then be re-privatised in the future. I expect that the taxpayer will make money out of the deal in the long term. In my view though, the government will not touch the shorthaul, it's not in their interest to compete on routes that are already served by British carriers. Longhaul that's different, they'll see the need for an intercontinental flag carrier serving destinations which other British carriers do not.
Unfortunately the slots do not belong to IAG to sell off. They operate them or they lose them.