Dying back to a natural colour - advice?

Since your hair is bleached I would avoid dye with peroxide in it. Have a look at the adore brand if semi permanent dyes, they're sold on beeunique and they have a lot of natural shades.

I recommend going to your hairdresser because getting ashy blondes right is really hard. Especially, if you want them to match your roots.

Adore spiced amber f.e. had khaki green undertones on my head (and I was not even white-blonde). This is, if you choose a color with cool undertones. Most Adore colors seem to have warm undertones, and I am not sure if that will fit you. Looks like a bit of trial and error (which is also not cheap, unfortunately).

Good luck!

I'd just have to add that your hairdresser will probably want your business, and I don't think hairdressers use semi-permanent dyes much - she'll use something with peroxide.  I agree with Marthakins that you should keep with non-peroxide options, but of course if you wish, see what your hairdresser says then make a choice.

I've had great ashy effects from semis such as Pravana Silver and Nirvel ArtX Grey, but my base colour is not bright white, so I'd also agree it might be difficult finding a semi to create the right ash tone.  There are some good browns that, if they're 'cool' enough, you might be able to mix with the Pravana or Nirvel to achieve an ash tone.

By the way, I love ash - and I think it would look great on you given your description.  Good luck!

Semi-permanent seems to have a unanimous vote then! My only worry is how will semi-permanent brown dye look on bleached hair as it fades?

Also whenever I have tried semi-permanent dyes they've faded really quickly for me (though that was with directions, manic panic and crazy colour, all of them various darkish purple shades, so it could've just been the brand) so I'm wondering how often I'll have to keep dying my hair. Any other recommendations for brands? I was thinking the semi-permanent nice n easy, their permanent dye was good when I tried it so I was assuming their semi-permanent would be good too?

You're right that all of the adore brand brown dyes seem to be pretty warm and I def want to stick with ash brown, I feel like it'll neutralise pink tones in my skin (and ash brown looks lovely to me ^^)

If you use an ash toned brown on bleached hair it will go green, or blue or some other similarly sludgey colour. You need some warmth in there, because all the warm pigment has been bleached out of your hair. So you can either pre-pigment by dyeing red before using your brown or use a warm brown.
Nice&Easy do a semi permanent line that is 28 washes (technically a demi) as well as a true semi line (they call it non-permanent, 8 wash) I would avoid the true semis for white hair, they are very blue or green based, as they are not designed for use over bleached hair. You can mix them with red or orange to get a natural shade, but they don't last very long IMO, Adore is a much better choice.
You can use a demi with a very low vol peroxide if you hair is in good enough condition. You don't need any lift, so I would swap the peroxide in the box for a lower vol, 5-10 vol is all you need for a deposit only colour. By using a lower vol peroxide the colour will be slightly darker on your hair, so pick a shade lighter than you want. Always strand test, as box dyes often don't indicate the underlying pigments used.

Ah right okay. I didn't think of how the tone would be affected by having bleached hair, thanks for letting me know! If I need a slightly warmer tone I'll try one of the adore shades then like you say, they seem to be recommended a lot. Is adore a demi or a semi dye? D'you know how long it usually lasts?

Adore is a true semi/direct dye. It will fade, especially if you wash with shampoo, but any colour will fade over bleached hair. IME direct dyes last better than permanent dyes on bleached hair.
I have found adores Mocha to be fairly neutral in tone, but remember you can mix these dyes to get the exact shade you want. Also because they are non damaging you can top the colour up as often as required.

Didn't you say to avoid true semis for bleached hair? or did you mean specifically nice n easy's semi dyes?

She meant the nice n easy ones because they're made for toning yellow hair.

Yes i just meant the Nice&Easy, they only work over natural coloured hair. Adore is much better for bleached hair, or Wella Colour fresh.