If your intending to marry in the UK, DO NOT apply for a Visitor Visa, otherwise you will have problems. Take note of the following;
If you wish to bring your girlfriend to the UK with the intention of marrying here, then you must do so under the appropriate visa. Do not try and bring her on a visitors visa, and then try and upgrade it, as they won't accept it, and you will then find it extremely difficult to get the immigration department to let her stay. Be totally honest with them from the start, as the know all the tricks, and if you show any signs of deceit with them, then any further applications will be denied.
I had the same idea as you a few years back, but was informed of what I am telling you, and have since come across people who can't get visa's now, after being less than honest to the immigration in the first place. Use the form that tells them she intends to live here, and take it from their. If she is successful you will need to marry within 12 months.
They are not totally heartless though if your honest with them, I was unable to marry within the 12 months due to illness at the time, but having supplied the evidence and explained my situation to them, they allowed a further 6 month extension.
