It doesn't say you have to live together - merely that you have to be married.To qualify for entry clearance or leave to remain as the Partner of a Relevant Points Based System Migrant, an applicant must meet the requirements listed below. If the applicant meets these requirements, entry clearance or leave to remain will be granted. If the applicant does not meet these requirements, the application will be refused.
Requirements:
(a) The applicant must not fall for refusal under the general grounds for refusal, and if applying for leave to remain, must not be an illegal entrant.
(b) The applicant must be the spouse or civil partner, unmarried or same-sex partner of a person who:
(i) has valid leave to enter or remain as a Relevant Points Based System Migrant, or
(ii) is, at the same time, being granted entry clearance or leave to remain as a Relevant Points Based System Migrant.
(c) An applicant who is the unmarried or same-sex partner of a Relevant Points Based System Migrant must also meet the following requirements:
(i) any previous marriage or civil partnership or similar relationship by the applicant or the Relevant Points Based System Migrant with another person must have permanently broken down,
(ii) the applicant and the Relevant Points Based System Migrant must not be so closely related that they would be prohibited from marrying each other in the UK, and
(iii) the applicant and the Relevant Points Based System Migrant must have been living together in a relationship similar to marriage or civil partnership for a period of at least 2 years.
(d) The marriage or civil partnership, or relationship similar to marriage or civil partnership, must be subsisting at the time the application is made.
(e) The applicant and the Relevant Points Based System Migrant must intend to live with the other as their spouse or civil partner, unmarried or same-sex partner throughout the applicants stay in the UK.
(f) The applicant must not intend to stay in the UK beyond any period of leave granted to the Relevant Points Based System Migrant .
(g) Unless the Relevant Points Based System Migrant is a Tier 1 (Investor) Migrant, there must be a sufficient level of funds available to the applicant, as set out in Appendix E.
(h) An applicant who is applying for leave to remain, must have, or have last been granted, leave:
(i) as the Partner of a Relevant Points Based System Migrant,
(ii) as the spouse or civil partner, unmarried or same-sex partner of a person with leave under another category of these Rules who has since been granted, or is, at the same time, being granted leave to remain as a Relevant Points Based System Migrant, or
(iii) in any other category of these Rules, provided the Relevant Points Based System Migrant has, or is being granted, leave to remain as a Tier 5 (Temporary Worker) Migrant in the creative and sporting subcategory on the basis of having met the requirement at paragraph 245ZQ(b)(ii).
(i) If the Relevant Points Based System Migrant is a Tier 4 Migrant, the Tier 4 Migrant must be applying 4 for or have entry clearance or leave to remain for a course of study that is longer than six months.
Providing Slovenia are ok with you doing a marriage - it should be fine.
You'll need to get your marriage certificate translated and certified into croatian and I'd suggest into english too.