Re: I wish...
Good idea in principle. The problem is that the average person in the street is incapable of budgeting for future outgoings. They see money in their pay packet and they spend it. When Gordon Brown cocked up Family Benefit payments (erroneously paying out too much and then having to reclaim it back later) it wrong-footed a lot of families because they took it on good faith (the guvment ne'er makes a mistook, right?) that this was their money to spend. Supposedly this money should have been ring-fenced by its recipients for essential outgoings but clearly this didn't happen in practice. My feelings at the time were that families affected by the mistake would paradoxically have felt better off if they had never had any of the money in the first place
UK Tax Codes indicate how much one is allowed to earn before any tax is payable, but the system doesn't trust you because it forecasts how much you are going to earn over the payroll year and that "allowance" is then divvied out to you in monthly/weekly dollops (depending on how frequently you're paid).
The allowance is a blanket one in that it is the same for the average person in the street and so is not designed to cater for all situations. With the advent of RTI it begins to be within the realms of possibility for HMRC to individually tailor people's tax according to their personal situation, but I'm sure their priorities are somewhat different.