I think a citizens' income would be a good thing.
Everybody gets a very basic standard of living, just sufficient to have food and shelter and basic clothing. The disabled, their carers, and pensioners, get additional.
But also reduce the zero-tax band to near zero. Why? A key thing is that all citizens should have "skin in the game", at the moment voters dependent on benefits don't care about government efficiencies, but if they realise they are paying for waste, they might start to care, and they have no reason to not be involved.
A simpler tax regime should be implemented, even a flat rate tax. Some people will say "but the rich should pay more". They do, it's a percentage, not an absolute amount.
Abolish NI, it's a pointless separation. I think should also abolish the abitrary distinction between salary, savings interest, and dividends. All these things make collecting tax more complex and expensive, and complexity causes loopholes which the rich can subvert.
The hordes of civil servants doing the administration of benefits and taxation will massively reduced. Ok, this will create a spike in unemployment, but the burden on the taxpayer will be reduced because they'll only be receiving citizens income rather than a full salary!
How many? There are 70,000 workers in HMRC, if that could be reduced to 10,000, that'd probably save about £2.7B. There's 45,000 at the DWP, cut to 10,000, that'd save another £1.6B. Total £4.3B
If the citizen's income was set at say £6000 a year, those savings would be enough for over 700,000 people.