Can my spouse and I split rental income to pay less tax?

Only if you actually own the property in a way that supports the split you're claiming — HMRC taxes jointly owned property 50:50 by default regardless of who manages it or whose name is on the mortgage, unless you're married or in a civil partnership and file a formal declaration (Form 17) with evidence of a different beneficial ownership split, such as 90:10.

Simply deciding between yourselves to put the rental income on the lower earner's tax return without the ownership and paperwork to back it up isn't something HMRC will accept if it's ever checked. Done properly, this can meaningfully reduce a couple's combined tax bill where one of you is a higher-rate taxpayer and the other isn't using their full personal allowance. It needs the actual legal ownership adjusted first (which may involve a solicitor) and the declaration filed correctly — it's not a same-day fix, so it's worth starting the conversation well before the tax year end you want it to apply from.