I've had good experiences with two purple toners - one by manic panic, I think it's called virgin snow, and one by guhl, to get rid of those kinds of tones (my hair is lighter than yours though). They're very similar products; this is what the guhl looks like over here, maybe you can get it in the UK, too - it's sold in run-of-the-mill drugstores over here, where the deep treatments are.
They're more like a deep treatment than a conditioner, if that makes sense - the effects last longer, you don't have to reapply every time you wash. That means you can also just apply them to the areas where you want the toner to go. The coloring stuff in them is the same as what is used in the purple/blue shampoos and conditioners, but there's a bit more of it, the color is a little stronger, and it lasts longer.
For me, I only had to use them a few times, then the color just stuck. That often happens with highlighted hair, if it's just been damaged enough but not too much, there's some room in the hair's structure for those kinds of color molecules to get stuck. I hope that makes sense.
In any case, I'd use something like that, because you can just put it where you need it and don't need to worry about it affecting your overall color like you do with shampoo and conditioner. I don't think that blue/purple shampoo/conditioner would do a lot to your overall color, it would probably show up at most in the sunshine as a subtle cast of some sort, but you know, just in case. When I used blue shampoos I did feel they made my whites look too gray over time (I try to pass off my whites as blond highlights - not sure that that works but I didn't like the gray cast the blue shampoo gave to them). But I don't know if you have grays.
ETA: If you can't find those toners, you can also just mix some blue/purple deposit-only dye (manic panic, stargazer etc) with a lot of deep treatment - that does the same thing. Blue has a stronger effect than purple, so you need to use less of it/mix more deep treatment in or you end up with blue/green tones (which is way worse than a bit of brass - way, way worse). Maybe start with something like a 1:20 or 1:30 ratio? I've done it and it worked, but I don't remember the ratio I used. I do remember that you need to dilute the heck out of the pure rainbow deposit-only colors to use them this way, they have a lot of pigment.