Don't ever let them tell you its an economic decision. Unless they can argue the fiscal multiplication of other investments is greater than public sector pay then any pay freeze at a time of increased spending for any other budget is most likely philosophical. Economics can be used to feed a political philosophy however seen fit.
At the route of all of this is a philosophical view of the public sector. I'm not talking full blown libertarianism, rather a softer version, attitudes towards what is perceived as public sector "largesse".
The BBC printed today that public sector pay is 7% higher than private sector pay. Thats a nice figure to try and justify a pay freeze but some public sector careers are a cul-de-sac. What I mean is once you're in there's no promotion, or very few opportunities for promotion, so a pay rise is all you have. The job role doesn't move much. The private sector, or specifically the service sector economy of cities, London in particular is a market place rather than a cul-de-sac, in a market place you sell yourself to the highest bidder. So people move jobs, roles, organisations freely. That's their pay rise. Actual private sector pay rises for being in the same role in most businesses is far worse than the public sector. Especially if you aren't unionised.
Tax doesnt pay for spend at the national level. It hasn't for a while. Recent theories are that it is a means to control inflation. We are now closing in on £1trn that the Bank of England has e-printed and given to the state for spending. The Cameron-Osborne government said £500bn ago it had to stop. Unless we have a miracle boom the debt the state has won't fall. To fall tax revenue needs to exceed both expenditure and annual debt interest payments. Paying down Bank of England debt means taking money out of the economy. With current inflation levels that will stunt growth at best, cause a recession at worst.
Public sector can definitely have a pay rise, it won't get it for philosophical reasons, or at best what might be called economic conservativism, which in real English means shitting yourself that it might rain one day.
You can argue the opposite, that a pay rise is also philosophical. Certainly giving someone more pay for a role in a pandemic is not an economic choice either.
One of my personal biggest gripes with the Corbyn-McDonnell era is their inability to repaint the picture on economic choice. They pretty much gifted the argument to the Conservative Party.