Petition tax allowance for dependents

There is a £6,439 tax liability difference between a couple with child that each earn £32,010 and a single mother that earns £64,020. Yet, now there are more single mothers or head of household mothers than ever. Why should these mum's be punished with a higher tax liability when instead they should be encouraged to continue working and be financially independent? These mum's are further punished if they live in London and have to pay nursery fees of up to £2000/mo. A single mother earning this much does not qualify for any sort of benefit and this is fine but I would like to petition a tax alloance for dependents. Thank you.