Seeing as we're on the subject of basic economics, let's discuss the source of this money tree shall we?
When the government borrows, it borrows from investors looking to invest. When the government borrows from an investor, it borrows money that would otherwise have gone to the private sector to create more jobs or products, the proceeds of which would then have been spent and increased growth. So the government isn't creating jobs or product, it's just shifting them from the private sector to the public.
The main investors in government debt are pension funds and insurance companies iirc. They were never going to take that money and put it into riskier, private sector ventures were they?
Likewise foreign central banks, they purchase gilts as part of their foreign currency reserves.
Then of course, there are the times when the BoE buys gilts. In 2009, the 3rd biggest holder of UK debt was the Bank of England. Again, this wasn't money that was ever going to go anywhere else.
Also, the government borrows money to spend it. The spending ends up in the private sector, surely? That's why government investment can boost the economy for everyone.
This means the government has to be at least as efficient and at least as good as a person or business at maximising the benefits of the spending - something that has never happened before.
In light of who holds government debt and why, I would disagree. See the previous points.
Even then, all we're doing is shifting jobs from the boom part of the cycle to the bust. The debt has to be paid at some point, it can't increase forever. So the growth of the economy would have to outstrip the interest rate paid on that borrowing or else we'll just be taking the pain on the next boom cycle. So we'd have to be very sure that these infrastructure investments will definitely pay off in terms of GDP, that they will increase GDP at more than the rate of interest and will have to have little or no future costs so that we can pay back the debt in the future. Again, not things that governments have ever done in the past. You have to stop thinking of government debt as a fixed rate loan. The bonds will have an expiry and that expiry will tend to be short when interest rates are low. Those bonds will need renewing unless we've paid all our debt, and they'll be renewed at the prevailing interest rate. Borrowing low doesn't mean it will stay low.
Don't gilts get paid back at a fixed amount, the rate of interest at the time of issue? As long as there is demand to buy them (and when there isn't, the Bank of England are willing to create it by buying them themselves) and the government can service the debt, then there is no problem. Borrowing for a program of investment that boosts growth and tax receipts doesn't seem like something that would put off the main bulk of the purchasers of gilts.
So what infrastructure that we need could we build?
I would hope that the government of the day could take a proper review and come up with detailed proposals.
I doubt we'd be allowed to put another 6 lanes on the M25 - the locals wouldn't like that. We need at least another runway at both LHR & LGW - can't do that it seems. We don't have the time or knowledge to build nuclear power plants, we don't have the technology to make renewable energy usable on a large scale. We need to completely rebuild all the railways but we've spent the last 200 years building right alongside them so we can't do much there. Training/education won't improve or pay off in the timescales we need.
See above.