married couples should have the option of being able to pool their tax allowances
Ok, why only married couples? What about unmarried single earner families? Doesn't that just give any potential (unfair?) advantage to married couples rather than couples without children, per the op?
What about a high earning single parent who loses CB and their personal allowance? Do we just give them their personal allowance and CB back to balance with a couple?
How does transferring allowances work? Joint election? Can one spouse make a unilateral election? Can a joint election be revoked unilaterally?
Is it all or nothing - can they specify part of an unused allowance or do they have to transfer the full amount or zero?
Do they have to decide before the start of a tax year? Can they later change their minds? Do they have to file tax returns to make the claim or to finalise the claim? Can they change their minds each year? Is a claim fixed once made? What happens if the higher earner loses their job partway through a year, can the claim be altered or revoked? Can it be prorated or will it be all or nothing?
What happens if they separate or divorce during a tax year? What happens to the claim? Can they still claim while separated but not divorced? Can they claim for part of the year or do they lose the claim for the whole year?
What about savings and dividends allowances? Should we make them transferable between couples too? Capital gains tax annual exemptions?
What about if one person is not entitled to a personal allowance because parliament decided individuals in their circumstances should not benefit from one - can their spouse usurp that by transferring theirs? Or will there be limits and exclusions?
If there are restrictions, how do we maintain independent taxation, bearing in mind HMRC can't disclose one spouse's tax affairs to the other? What happens if a lower earner transfers their personal allowance to a higher earner who should not have been eligible for it because they didn't know how high their income was, will there be penalties?
What if one or both individuals are not UK domiciled for tax?
Do we also pool the tax rate bands between spouses? Can they juggle those about too? So if you've not used all your basic rate band, can you extend your higher earning spouse's basic rate threshold?
What about reliefs for gift aid donations or pension contributions, should they be able to transfer the benefits of those? If your spouse runs a business and makes a loss, should they be able to transfer that to you to offset your income?
Where are we drawing the lines? And what lines even are we trying to draw when we talk about pooling allowances?
And where are all the staff at HMRC to deal with something this complicated and advise all the confused people and deal with all the mistakes caused by that confusion? The high income child benefit charge has been enough of a nightmare.