I think the long and short of it is corruption, really. On one hand, the present government is completely addicted to outsourcing and consultancy, on the basis that the private sector is supposedly more 'efficient' than the 'bloated' public sector, whereas in fact the competition that would notionally keep prices low is often non-existent, and contracts get handed out with little if any proper scrutiny, and usually to whoever has offered or already given the biggest party donations, or to people who are the partner, sibling, or old school chum of the MPs making the decision (if they're not in fact the same person).
And on the other, once the contract is signed, the contracted party often has such favourable terms that it can basically string the construction process out indefinitely and still get paid, regardless of how outrageously late or over-budget it ends up being. The deal the government signed with EDF to build the Hinkley Point C nuclear plant is a particularly egregious example,
as it's likely to be over a *decade* late, but the government is legally compelled to keep handing over cash regardless. So there's no real incentive to actually stick to the budget or schedule at all.
The last Labour government was bad enough for this sort of thing, but it's really gone into overdrive with this lot, to the extent that the construction industry has been called a wing of the Conservative party (or maybe even vice versa).
https://www.theconstructionindex.co.uk/news/view/tories-buouyed-by-construction-donations
