Colouring over bleached hair

That's a good point Jules, about both.  As an added bonus, pigment depositing shampoo sticks wonderfully to bleached hair.

Coloring over bleached hair is hard, it will come off eventually, and if you go red your hair will likely fade to peach, and I'm not sure there's much you can do about it, except redo it frequently.  Your hair is just very porous after it has been bleached, so dye slides off and reds slide off quicker than most dyes.  Using a protein filler first before dying it should help some.  If you can't acquire a filler to do it yourself in the UK, they should be able to do that at a salon. (In the US we get that stuff at beauty supply shops, and I thought I remembered someone in the UK saying they couldn't shop there).  If you're going to do it yourself, be sure to test strand because if your hair is white enough, the color will likely come out very bright.