Dyed brown hair desperate to go back blonde!!

Prelightener is bleach.  Blonde hair dyes are bleach.  Highlights are strong bleach.  You really don't want to use them any closer than one month apart.

If Colour B4 didn't do much for your hair, your only option left is bleach.  You can't go from dark brown to blonde overnight without completely destroying your hair; it is a long process if you want to maintain your hair's integrity and health.  Your options now are to either use a full bleach, once every 4-6 weeks, or bleach bathe once every 1-2 weeks, and you'll have to live with varying shades of orange in between.  You can tone the orange to browns, but sometimes toning orange shades will give a muddy khaki colour.

I would give a color remover another try again and be sure to apply it quickly, right after mixing it, thoroughly, and be sure to rinse very well, for about 20 minutes, dry it, then rinse for another 20. If it still doesn't do anything you may just have the type of hair that just doesn't work well with color removers unfortunately. I would try it again though, I've used them before and not had much of a result, but then I used another one and it lightened quite a bit. I think it's worth the money and time if it can potentially save your hair from unnecessary bleaching/damage. The worst that can happen is that it won't work and you'll be out the money for it.

I agree with Intemp though, if that fails then your only other option is to bleach it, and I would try to bleach it slowly to avoid more damage.

If the color remover doesn't work, before you bleach, I would try doing a few vitamin c treatments though. They are intended for fading semis, but I have seen then lighten dark brown hair to a light brown/dark blonde, on hair that was previously pale yellow.

Thanks for your suggestions!
I decided to try colour b4 one last time the other day like you said and it actually worked - before I used it this time I tried not to use any straightening or curling irons on my hair and also a day before I shampooed a few times with some anti build up shampoo. I kept the colour b4 on for about 90 mins and it took me down to a dark/ medium blonde colour with a tad ginger tint so today I put some purple shampoo on and now back to my original dirty blonde colour, so relieved I didn't need to use the bleach in the end!

Thanks for your help!! 🙂

Yay I'm so glad it worked out well for you 🙂

What brand of permanent dye were you using? I have natural blonde hair. Had 6 months of Red, Magenta, Copper, Copper Red. I used 2 bottles of color oops and it removed all of it. I later bleached my roots when they were ready a few weeks later. I did one more bleach to my lengths (20 Vol only).  Now I am platinum again waiting to tone or use purple conditioner/poo for a bit.  I did this same process February last year and it worked fantastic.