"without paying hundreds of millions of pounds in compensation to Royal Mail" < the Royal Mail makes about £3m/year in profits from selling the data. It would cost a lot less than hundreds of millions to bring it back into govt
They made about £18m in revenue though. It depends who'd be taking on the costs. If they still need to maintain the PAF (they need it themselves), then it's the revenue that would need replacing. The best solution would probably be to say that the OS will take over the maintenance (presumably funded by central government if it's going to be open data), and then Royal Mail will have access to the data and can be paid a lot less in compensation.
