Under the current tax system a person working 20 hours a week at £20 an hour and a person working 40 hours a week at £10 an hour have to pay the same amounts of income tax. This is despite the fact that the person working 40 hours would have additional costs in the way of commuting and quite often childcare as well as much less available time for other activities. The current tax system also leads to the situation that for many overtime would be taxed at such a higher percentage to not be worth doing which can often lead to short-term staff shortages in essential public services (quite recently seen in train drivers).
By moving to a system based upon the hourly wage the government could ensure all work at minimum wage wasn't taxed, while those who choose to work very part-time in well paid roles are taxed equivalently to those who do the same job full-time. This would increase the incentives for overtime and full time work amongst skilled workers many of whom have skills the country is short of while at the same time remove many who have to work long hours on minimum wage from paying income tax. A system based on hourly earnings would also help prevent the perverse situation in which part time work can lead to a higher disposable income than full time work due to take home pay not increasing as much as additional costs due to high marginal tax rates.