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 £12,570 for 2021/22), to their spouse or civil partner. The recipient of the marriage allowance must pay tax at no more than the basic rate of 20%.
When claimed, the marriage allowance is worth £252 for 2020/21.
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.