Marriage Visitor Visa then spouse visa?

The current version of the visit guidance does not contain the requirement "you’re not planning to stay or settle in the UK after your marriage or civil partnership" that is expressed on the public information page for the marriage visitor visa. That phrase is merely intended to underline that the visa is not intended for those who want to enter the UK for settlement. It does not rule out future settlement in the UK.

The visit guidance and the immigration rules on which it is based have a different requirement: the visitor must be a "genuine visitor," which means among other things that they will "leave at the end of their stay." This is far different from having no intention (ever) to settle in the UK.

Immigration rules Appendix V has, at part V6, the following:

V 6.1 An applicant for a marriage or civil partnership visit visa must satisfy the decision maker that they meet the requirements at V 4.2 - V 4.10 and must be aged 18 or over.

V 6.2 On arrival in the UK a visitor coming to marry or form a civil partnership, or give notice of this, in the UK must have a valid visit visa endorsed with this purpose and the name of the holder’s fiancé(e) or proposed civil partner.

Additional eligibility requirements for a marriage or civil partnership visit visa
V 6.3 An applicant seeking to come to the UK as a visitor who wishes to give notice of marriage or civil partnership, or marry or form a civil partnership, in the UK during that visit must satisfy the decision maker that they:
(a) intend to give notice of marriage or civil partnership; or
(b) intend to marry or form a civil partnership; and
(c) do not intend to give notice of or enter into a sham marriage or sham civil partnership, within the validity period covered by their visit visa.

Paragraphs V 4.2 - V 4.10 are the standard visitor paragraphs. Do they prevent someone who wants to settle in the UK in the future from visiting? The paragraphs create the following requirements:

• V 4.2(a): will leave at the end of their visit.
• V 4.2(b): will not live in the UK for extended periods through frequent or successive visits, or make the UK their main home
• V 4.2(c): is genuinely seeking entry for a purpose that is permitted by the visitor routes (these are listed in Appendices 3, 4 and 5)
• V 4.2(d): will not undertake any prohibited activities set out in V 4.5 – V 4.10
• V 4.2(e): must have sufficient funds to cover all reasonable costs in relation to their visit without working or accessing public funds. This includes the cost of the return or onward journey, any costs relating to dependants, and the cost of planned activities such as private medical treatment

None of this implicates future plans to settle.

Paragraphs V 4.3 through V 4.9 concern unrelated matters:

• V 4.3: third-party sponsors
• V 4.4: written undertakings by third-party sponsors
• V 4.5: intention to work is prohibited
• V 4.6: permitted activities may not include work
• V 4.7: certain payments from UK sources are permitted
• V 4.8: intention to study is prohibited, with exceptions
• V 4.9: intention to undergo public medical treatment is prohibited with exceptions

The inclusion of paragraph V 4.10 in paragraph V 6.1 is somewhat mysterious, since 4.10 says

V 4.10 The applicant must not intend to marry or form a civil partnership, or to give notice of this, in the UK, except where they have a visit visa endorsed for marriage or civil partnership.

Taken together, the two paragraphs somewhat oddly say that

An applicant for a marriage or civil partnership visit visa must satisfy the decision maker that they do not intend to marry of form a civil partnership, or to give notice of this, in the UK, except where they have a visit visa endorsed for marriage or civil partnership.

But that is just a byproduct of the UK's byzantine immigration law drafting, and has little direct bearing on the present question.

The main point is that there is nothing here to prevent someone with an intention to settle in the UK at some future time from using a marriage visitor visa to visit the UK, get married there, and then leave.