The last Labour Government, trust me, spent ££££££s remedying years of neglect to school buildings. They had a plan to rebuild/replace all unsuitable schools over X years, but as has been noted above, Gove scrapped it. The pre-1997 Tory Government was parsimonious, to put it mildly.
Another problem has been the tendency to devolve more or all of what money there is to schools. School Governors tend to prioritise tarting up the foyer, or the staff room, above stuff that can't be seen but which needs to be done. When all schools were under local authority management, the spending was prioritised, with most spent on the most decrepit schools and on the most essential work. This is no longer the case and hasn't been for about 25-30 years.
As a side issue, many of these schools will still have asbestos in them. It's safe while it's sealed in, but the minute you start knocking the building about with major works it adds all manner of complications and extra costs. It's a fucking minefield.
At some point this country needs to face a hard fact - you cannot run essential public services on the cheap and so-called 'savings' are almost always a deferral of expenditure, the equivalent of not having your car serviced for five years and then being surprised when you get a big bill when it eventually breaks down.
The current fragmented education system, with various tin pot 'academy chains' and 'free schools' is a recipe for unorganised chaos, and will make this problem much harder to solve than it would have been 35 years ago.