Fiance of an UK Citizen, permanent resident or EU Citizen

The requirements for immigration to UK as a fiance your find are very similar to the rules for married couples. You may wish to consider very carefully whether it is best to marry first and then apply for entry as this will avoid the necessity of making a fiance application and then a second application to remain as a spouse. However, please note that it is no longer possible for most visitors to change status to a spouse visa. The basic requirements for immigration to UK are as follows:

You are granted entry to the UK for a period of six months during which time you must marry your fiance in the UK. If there are exceptional reasons for delaying the marriage until after the initial six months an extension to stay in the UK may be granted.
You will not be permitted to work in the UK on the fiance visa and you and your fiance must show that there will be sufficient funds and accommodation in the UK without requiring public funds before and after the marriage. Public funds cover various benefits paid by the Government if you are currently looking for work, if you are on a low income and if you are in various other situations.
You must apply for a marriage visa from within the UK when you are married, and will then be eligible to work freely in the UK.