Hi,

If she is on the holiday in the UK (visitors visa), you can not get married!!!

This is what you should do!

The requirements for immigration to UK as a Fiancé/e’s 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/e 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:

Unless you are engaged to an EU Citizen (not British citizen in this situation!) with a five year residence permit you must apply for entry from outside the UK. If you wish to apply from within the UK in other circumstances your application is likely to be refused unless you can show exceptional reasons why the application should be granted.
You are granted entry to the UK for a period of six months during which time you must marry your fiancé/e in the UK. If there are exceptional reasons for delaying 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/e visa, and you and your fiance/e 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 have met your future spouse. This is to prevent a situation that occurs sometimes in arranged marriages where the future husband and wife have never met.
You must intend to live together permanently with your future spouse once you are married.
It is a requirement that the overseas fiancé/fiancée has to be 16 years old or over, and the sponsor in the UK has to be at least 18 years old.
Children of the overseas fiance/e who are under 18 years old are only allowed entry to the UK as dependants if it can be shown that it would cause serious problems if they are not allowed entry at the same time. Therefore this sort of application will be difficult to make, and in many cases it may be best to wait until after the marriage and then apply for entry of dependents.
You should apply for a marriage visa from within the UK when you are married, and will then be eligible to work freely in the UK.

Regards

Triple H