The main reason for the increase in debt under Labour was the banking crash, not overspending on other matters. Since 2010 and "fiscal conservatism", debt has remained high despite wage restraint in the public sector (0% or 1% deals every year for most), despite the sell-offs of tens of billions of pounds worth of state property and
despite rising taxes. The coalition government had a bigger deficit every year of its existence than the pre-crash Labour government did.
As for the "magical money tree", in many cases - especially with regards to PFI projects and outsourced bits of the state - spending now to take them over would reduce in a lower total cost to the taxpayer; when Boris (!) took the Tube PPP back in house it was estimated he saved £3.6 billion.