I would really love to know more about what sort of market research they did on this ad.
I imagine it was probably more qualitative than quantitative, as they would have to have shown respondents the ad in order to get feedback on it - so small samples, probably focus groups. I also imagine they probably would have wanted to screen respondents as to their likelihood of shopping at HN - so within this already small sample, there would have been quotas for existing HN-shoppers, and possibly also for people who may not necessarily have shopped there before, but who'd be open to it.
I imagine they probably would also have screened respondents for their social demographic - income, age, etc, etc.
There's no point them forming a focus group consisting of people that the ad doesn't target. The opinions of a group of middle-aged men, for example, aren't going to be relevant, are they? They're not trying to sell party dresses to middle-aged men.
So, I imagine, the focus groups would be made up mainly of existing customers, as well as women people who have not yet shopped there, but who HN would like to shop there - after all, it's all about increasing sales. So obviously people above a certain income bracket.
Now - imagine the sort of feedback this group of people would give that ad. A big thumbs up for the most part, no? They probably identify to a certain extent, with the woman at the end. Of if they don't they wish they did. They aspire to be a hell of a lot more like her than the women in the first half of the ad.
Had they conducted focus groups with an actual representative group of adult human beings, I'm sure their findings would have been very, very different. Probably more along the lines of the mixed responses to this thread. Probably even much more disparate, since this thread/forum is populated mostly by women, many middle-class, and for the most part old enough to be mothers. In other words, with the bulk of their partying days behind them; not in the thick of nights out and lost weekends, responsibility-free.
I remember the 'walk of shame' from my days, and associate it as much with just being out all night, as following on from a one-night stand. Nothing to actually be ashamed of.
I think they had a good idea, but the execution is kind of abysmal, as it has come across as classist, sexist, fattist and judgy/disapproving. In my day, walkers of shame got loads more nudge, nudge, wink, wink looks as opposed to sideways, disapproving looks but they obviously couldn't incorporate that into the ad, as it detracts from the idea that these women should be feeling awkward, embarrassed, ashamed of themselves. And would only feel better if they were wearing a posh frock.
If you think about it, there is no way to do this ad that does accurately portray the 'walk of shame' - which is actually a bit of a sniggery, what-am-I-like? laugh; not something to actually be ashamed of. 
Because if you did portray it like it actually is, with the passers-by giving the girls sly smiles instead of judgy looks, then there would be no reason for women to go out and spend £££ on a dress.
To my mind, it's an ad man's idea of what 'the walk of shame' must be like for a woman, as opposed to a woman's real-life experience of what the walk of 'shame' is (or was, back in the day!).
Oh, and the ad is also off-target for another reason, since their market probably is more women of our age - older, with more disposable incomes, etc. But, our 'walk of shame' days are well behind us. How many footloose and care-free single teens and 20-somethings can afford to splurge big bucks on a dress from Harvey Nicks? They wish...! 