The UK tax system is based on the independent taxation of each individual so it is difficult to reward couples for being married, but the marriage allowance attempts to do that.
It is a transfer of 10% of the personal allowance from the partner who does not have sufficient income to use their full allowance (currently £12,500 for 2019/20), to their spouse or civil partner. The recipient of the marriage allowance must pay tax at no more than the basic rate of 20% (21% in Scotland).
When claimed, the marriage allowance is worth £250 for 2019/20. If you have claimed the marriage allowance for all years from its introduction in 2015/16 you will be £900 better off. The deadline for claiming the marriage allowance for 2015/16 is 5th April 2020.
You can claim it online: www.gov.uk/apply-marriage-allowance; by telephoning HMRC: 0300 200 3300; by writing a letter; or in a tax return.
If you were widowed in the past four years, you can still claim the marriage allowance for years in which your partner was living from 6th April 2015 onwards.