Can someone please tell me what colour I need to get to make buttercream white? Which is the best brand I can buy in the UK?

There is nothing you can mix to make white. It is the absence off all color. You have to start with white. Make your icing using all white ingredients to get white.

For a pure white frosting, use clear vanilla and no butter. I do not use butter at all (I live in Arkansas with heat and humidity), I only almond and vanila, but some people use a clear butter flavor or emulsion. I personally hate the fake butter flavor

You can add a tiny touch of violet gel paste (start small, you can add but can't take away) to balance out the yellow of butter (if you are using a butter recipe).

If you want a pure white colour you need to use a shortening (UK brand is called Trex) based recipe.

You could try use bright white powdered colour in it, but I have never added that to buttercream so not sure if it would work.

.......There is nothing you can mix to make white. It is the absence off all color.............

True that white is the absence of all color BUT.............
Ever hear of blueing? *Years* ago it was a common additive to white sheets etc when washing clothes.
Decorator CAN use a *EVER SO LITTLE BIT* of blue or purple color to off set the butter color.

Also, the more you blend the shortening and butter the whiter the color. Beat (but on a slow-ish speed) your butter/shortening together well - like up to 10 minutes, then after you've add all ingredients again, slow speed beat your icing another 10 minutes or so. You will see an imporvement in how light it gets and not so yellow.

You can whiten buttercream with white food coloring. I haven't had to use it yet because my frosting has been white enough...and I use real butter, but my recipe has cream cheese in it and when they are creamed together, it gets fairly white. But the bakery I worked at used the same recipe and sometimes they would need to whiten it so they would add I believe CK gel white. I think Americolor gel has a white too.

One thing the bakery did (and I do) is layer the regular non-colored butter cream on first and then refrigerate until hardened and then the last couple of layers add the colored BC so you won't have a 'colored frosting' taste.

Oh...and I've heard of people using a pin prick of violet to regular BC to whiten it.