I think that grades are useful if you want to go into teaching, classical playing [such as with an orchestra], possibly jazz [because learning fiendishly complicated theory would be advantageous] and math rock or tech-core for the same reasons. Grades might also be useful if you wish to compose for soundtracks or adverts, because a. you might be working with a whole bunch of instruments and a sound foundation of theoretical knowledge could prove extremely helpful, and b. the kind of corporate people who employ you for this sort of thing are likely to value what's written on a piece of paper.
However, for most other genres, contemporary, and recreational playing, I'd say that grades are unnecessary. They are, after all, just a label for your talent, not your talent itself. With guitar especially over any other instrument, it is entirely possible to learn everything you need via a bit of online research and plenty of personal persistence. I know very few guitarists with gradings [excepting the odd music teacher] and I've never been in a band where members have even asked about, let alone expected guitarists to have gradings - not because they weren't good enough, but because it simply seems to be uncommon/ unnecessary.
Since you currently have GCSEs coming up, I'd recommend waiting until the Summer if you do decide to take gradings, because frankly you'll have little enough free time as it is, and you want your guitar playing currently to be an escape from studying, not a return to it - that's just my opinion of course. Longer term, whether you grade or not depends what you want from your music, I'd say. It's not like you have a deadline though, after all - you can try for grades whenever you decide to do so.
