If you are going to be getting married to a foreign national and wish to take her back to the UK then she will need to apply for a UK Fiancee Visa to enter the United Kingdom. These are the requirements for the UK Fiancee Visa for your fiancee or girlfriend that you are wanting to bring back to the United Kingdom.
Note that if you are going to the UK to get married but the person is not a UK national then you need to apply for a UK Marriage Visit visa. This visa will allow you entry into the UK to get married to your fiancee who is not a British national and might be in the UK on a UK Student visa or a UK work permit. Once you get married then you have to leave the UK. That visa is only valid for 6 months. See the page on that.
Your fiancé(e) or proposed civil partner must be:
a British Citizen in the UK; or
present and settled in the UK.
You must show that:
you and your partner must be aged 18 or over at the date of application;
your partner must not be related to you in a way that means you could not marry in UK law;
you and your partner must have met in person;
your relationship must be genuine and subsisting;
you must be seeking entry to the UK to allow your marriage or civil partnership to take place;
any previous relationship must have permanently broken down;
you and your partner must intend to live together permanently in the UK;
you must meet the financial requirements; and
you must meet the English language requirement.
are a national of a majority English-speaking country; or
have passed an English language test approved by us at the appropriate level; or
hold a degree that was taught in English and is equivalent to a UK bachelor’s degree or above.
2 recent passport photographs and your passport
evidence of your age and your partner’s age
official evidence that your and your partner’s previous marriage(s) broken down permanently
evidence that you intend to marry or register your partnership (usually 6 months)
evidence that you have met each other
evidence that you intend to live together permanently after you have married