Normally the rules is that income is split 50/50 but there are some special rules for FHLs
https://www.gov.uk/hmrc-internal-manuals/property-income-manual/pim1030
Jointly owned property: husband & wife or civil partners
Husbands and wives or civil partners living together should generally be treated as entitled in equal shares to income from jointly held property.
See:
ICTA88/S282A for years up to 2006-07, and
ITA07/S836 for 2007-08 onwards.
However, this rule will not apply in any of the following instances:
# the income is earned income (or, like furnished holiday lettings, treated as earned income); ICTA88/S282A (4)(a), ITA07/S836 Exception D,
# there is actually a partnership; ICTA88/S282A (4)(b), ITA07/S836 Exception C. In this case the income is divided according to the terms of the partnership agreement,
# both husband and wife, or both civil partners, have signed a declaration under ICTA88/S282B or ITA07/S837 stating their beneficial interests in both the property and the income arising from it. However, a declaration is only valid if their interests in the income and in the property itself correspond.
Further guidance can be found at TSEM9800 onwards. Any problems about joint ownership which cannot be dealt with by reference to the TSEM guidance should be submitted to HMRC Trusts Bootle.
https://www.gov.uk/hmrc-internal-manuals/trusts-settlements-and-estates-manual/tsem9820
Property held jointly by married couples or civil partners: The 50/50 rule: Income from furnished holiday lettings
The 50/50 rule does not apply to income arising from a UK property business which consists of, or so far as it includes, the commercial letting of furnished holiday accommodation.
# If a spouse or civil partner carries on the activity alone: that spouse or civil partner is taxable on the income.
# If a spouse or civil partner carries on the activity with others: the income is split for tax purposes in the way the parties have agreed to split the profits amongst themselves.
