S1.3 immigration rules para 289AA - 295
(ii) the parties to the proposed marriage or civil partnership have met; and
(iii) each of the parties intends to live permanently with the other as his or her spouse or civil partner after the marriage or civil partnership ; and
(iv) adequate maintenance and accommodation without recourse to public funds will be available for the applicant until the date of the marriage or civil partnership; and
(v) there will, after the marriage or civil partnership, be adequate accommodation for the parties and any dependants without recourse to public funds in accommodation which they own or occupy exclusively; and
(vi) the parties will be able after the marriage or civil partnership to maintain themselves and any dependants adequately without recourse to public funds; and
S6 defines "present and settled"
Settled' is defined in paragraph 6 of HC395 and means "free from any restriction on the period for which he / she may remain in the UK, and ordinarily resident in the UK".
Fiance visa: The law does not state that after marriage the couple must remain in the UK.