I took my near-black color processed hair all the way to a light golden blonde over the course of three days, but in the process I damaged it pretty badly.
It is possible, but you really need to do your research. I read this
blog for tips, etc.
I'll tell you the process I went through, although parts may not be applicable. I went to a beauty supply store (like Sally's - idk if there is an equivalent in the UK) and purchased a couple of bottles of developer (volume 40 for maximum lift/lightening and volume 20 for minimal lift). I also purchased several packets of powder lightener and a deep treatment conditioner.
I used one packet of lightener at a time and did several courses of treatment, conditioning thoroughly each time. My hair did go orange at one point in time, but I knew it would. Once I got it to a point where I felt my hair was light enough, I purchased a couple of bottles of toner. Since my hair had strong yellow-orange undertones, I purchased toners with a blue-violet and violet base. I applied the blue-violet toner first, followed by the violet toner. As my hair still was not quite the color I wanted, I then purchased a tube of hair color (a light golden blonde) and a volume 40 developer (to lift the color just a bit more). I waited to purchase the toner because I wanted to see what undertones my hair would have, as it turned out, it was yellow, so I purchased a violet toner.
As you can see in my avatar, I have platinum roots with light golden blonde body. But again, my hair is quite damaged, and it's taken me about three weeks to get it mostly healthy, although definitely not back to normal.
I will say, your hair doesn't look as dark as mine was, so you likely won't have the problems getting your hair to lift. I would suggest doing this over a weekend so you have time to achieve the color you want, or at least have a color you wouldn't mind going out into public. My biggest tip is do your research!