If you are applying for a Student visa, it is very important that you do not travel to the UK before your Student visa is granted. This would be possible if you are a non-visa national as you could enter on your passport to visit for six months, for example, if you are a national of the USA, Canada or any EU/EEA country (as well as many others).
The problem with doing this is that your Student visa is not 'activated' until you enter the UK after the issue date. If you enter the UK before the issue date of the Student visa, it will not be valid, and you can only stay in the UK for six months. We would not be able to register you on your course if this is found to be the case as your Student visa will not be valid. The only solution would be for you to leave the UK and re-enter on your Student visa, which would likely be expensive and inconvenient.
If you are granted a Student entry clearance visa overseas (the vignette in your passport), you must ensure that you only enter the UK during the validity period of this visa.
If you are an EU/EEA national, please also ensure that you do not travel to the UK before your Student visa is granted (although this will be in a digital format).