It wont happen Adam, with the changes in the living wage even low paying employers will be paying wages of £20k or more in a few years, even now its not hard to find a paye factory job at around the £25k mark, add holiday pay, sick pay, pensions, expenses, and your talking at least £35k shining windows to equal it, there would be no incentive left for any sensible shiner to pay an extra 20% in tax.
£42k a year to equal a £25k paye job, I don't think so.