But the UK's debt - of around 90% of GDP is lower than most comparable countries
I don't know where you get your figures from but that is nonsense. According to the OECD our government debt as a proportion of GDP is higher than Australia, Austria, Belgium, Canada, Denmark, Finland, Germany, Netherlands, New Zealand, Norway, Spain, Sweden and Switzerland amongst others. It is above the Euro area and on a par with the US (which has the advantage of having the world's reserve currency).
Investment is a good thing to borrow for if you can afford it. It is not a good thing if the result is to push your borrowing costs beyond what you can afford. Both businesses and countries have gone bankrupt through borrowing more for investment than they could afford. And of course the more you borrow the higher the interest rate goes as the lender is concerned that you will be unable to service your debts. That applies for countries just as much as anyone else.
I also don't know where you get your figures for the changes to CFCs and LLPs. The CFC changes cost £1bn a year according to the treasury. I haven't seen any independent analysis that suggests it will be significantly higher than that, let alone as high as £50bn. The LLP changes will plug existing loopholes and should therefore raise revenue.
Roughly 10p in every £1 we pay in taxes goes to pay interest on the government's debts. That is why none of the major political parties shares your view that we should push borrowing even higher.