The solution is root and branch reform of the tax system.
The whole concept of tax credits -- charge people too much money and pay it back later -- is fundamantally broken.
I suggest replacing the present income tax system with its archaic bands (designed to make it feasible to work out everyone's taxes in the days before computers; the real bottleneck in the process, though, was the stupid pounds/shillings/pence system that persisted far too long) and personal allowances by a simple quadratic regression. That is, your tax y would be worked out from your salary x according to an equation of the form
y = a * x ** 2 + b * x + c
where a, b and c are constants determined from time to time by the Government.
The squared term removes the need for banding and so eliminates a popular tax avoidance strategy. The proportional term would dominate, of course. To provide for individual allowances, c would be made negative and if y < 0, then you don't pay any tax.
Now we have computers, it's not as impossible as it might have seemed in the early days. Actually, as long as you're using decimal currency, it's even doable without a computer.
I further suggest a phased introduction over 5 years; with everyone paying 80% of their taxes worked out according to the "old-style" method and 20% of their taxes worked out according to the "new-style" method in the first year, 60% + 40% the second year, &c. This will mitigate the effects of a drastic step-change in the tax regime.