This is such an important issue.
Sorry this is so long and if this repeats other suggestions - haven't yet read back through thread.
There's an election coming and Mumsnetters are clearly regarded as a target group. So get all the party leaders signed up - no such thing as a free biscuit. Better still, get them all to do it jointly - for hey look we can rise above petty difference brownie points.
Get the organisation's attention at the highest level. Get the Chief Execs of M&S, Asda, Tesco in for a webchat. If they have daughters or granddaughters don't be afraid to ask them where they drew/draw the line. If they exist, have a few examples from their empire to hand. "Chief Exec of X says high heels OK for 3 year olds" is not a headline they want. And if they do say it, make the sort of fuss which will get picked up more widely.
Contact the british retailers association, if that's the name, and ask them for a response.
Do collect as many hard examples as you can - pictures at minimum - of what the problem is, with as much specific detail as possible of where it was on sale and when. Put out a call on Mumsnet - give us a job, you have an army of photographers. No pix of other people's children, obviously.
I'd guess from what I see that quite a lot of the most inappropriate stuff is not bought by parents but by other adults (troubling thought that some know exactly what they are doing ) and gets worn just because it's in the house and the parent is too stressed, distracted, short of clothes to do anything about it. Or because the child gets their hands on it before parent has a chance and the fight to get it off after that them is too much. So an important target audience here is friends and relatives, especially maybe who buy things on an impulse because they think it's a bit of a laugh or a bit cute. Any publicity you can get somehow needs to attract their attention too, in a way they won't find snobby/prim - starting from the assumption they care about the child as a person..
Somewhere in here's a basic message about respecting children as people, not vehicles for a joke.
Hate to say it, but worth speaking to organisations that work with sex offenders and with vulnerable children to check whether they are picking up any indications that this sort of clothing is being added to the list of excuses for abusing children. If not, well that's one good thing. If so, though you might not want to wave that around, it would be worth putting to retailers and politicians privately. I doubt it's ever the real reason, and is no more than another excuse - but in any case we should ask why would anyone want to help create a climate where people who want to harm our children see their attitudes apparently affirmed and normalised in mainstream life?