Hey Willowwitch,
As reenie said, you can change your name at any time and you don't need to have a GRC to do that.
Have a look on the Gender Recognition Panel website for full information about the GRC
www.grp.gov.uk
You need to have lived in role for 2 years and have two medical reports from doctors before you will be given a certificate. You will then be recognised in the new gender for all legal purposes including marriage, pensions, etc. Until then you are still recognised by the law as being your original gender. So basically for those two years between transitioning and getting a GRC trans people are basically in a state of legal limbo. You are living as one gender but legally recognised as the other. However you can still change much of your documentation such as drivers license, passport , bank cards etc at the time of transition and that makes things much easier.
Good luck,