Harriet Walker: You can do a roaring trade in selling posh people the lives they feel they were cheated out of

A while ago, a non-posh friend asked me why posh people love dressing up so much. Just to be clear, she was asking me this not because I have some Roman numerals after my name, but as a fellow non-posh person.

We decided that fancy dress is just an easy way to fill up all the time one must have when one isn't worrying about money or loading the dishwasher oneself. But I can further add to the debate now, having just spent a weekend at what can only be described as a big fancy-dress party for posh people. It was billed as a "festival", but those normally have acts and activities. The only thing to do at this festival (it was 1920s-themed) was sit on the lawn in your finery and drink.

I'm not against sitting and drinking, not at all. It's actually one of my favourite activities. But the drinks at this place were so extortionately priced, as the tickets had also been, that I had felt – to use a non-posh phrase – a bit mugged off. (I was at a posh person's hen do, before you start asking me why I went.)

I was struck by how posh people like not only to dress up, but to dress up as times gone by, when their ilk was in charge. The good old days. Like the 1920s, or like my posh friend's posh parents who go to parties at their sailing club with titles such as "Last Days of the Raj".

Posh people like to dress up because they are misty-eyed about when times were better. Unlike the rest of us, who are showering off the proverbial mud from when we dragged ourselves out of the primordial soup and into semi-detached houses.

I have never seen so much glammed-up hair or so many fawn-like hunger thighs as I did at this 1920s party. And I work in fashion. There were flappers and headdresses, RAF uniforms and cheroots. The boys were as costumed as the girls. And, in what was just about the only historically accurate part of the event, everybody was white.

Turns out you can do a roaring trade in selling posh people the lives they think they were cheated out of by things such as social mobility, the General Strike and the NHS. You can sell them, as the jokers who ran this "festival" did, bottles of wine clearly marked with a Tesco Finest label for £35. You can make them pay £80 for the relief of only hanging out with other people like themselves.

We were less easily bought.

To counter the fact that you weren't allowed to bring your own booze, the hens I was with had clubbed together to buy something called a "Wine Rack" from a dark recess of the internet.

A Wine Rack is like a Wonderbra, but where the padding would normally be, there are two empty bladders which can be filled with alcohol in order to sneak it into over-priced events. A tube then protrudes out of the wearer's skirt for all to partake from. I got the short straw and was the one who ended up wearing it. I'd like to say this was because I'm known among my friends as party central, but in actuality it was because they were all blessed with such enormous posh bosoms that I was the only one who fitted into it.

That was the other thing. I was struck by how easily the posh attendees assimilated the period garb: they were built for it, they had evolved specifically to be in that role of louche sunny-afternoon debonairs, reeling from opium over the croquet hoops. I, on the other hand, do not have a vintage body. Things from the past look bizarre on my frame. I am tall and boxy, with a torso that speaks of manual labour rather than generations passed on a chaise longue.

By then end of the day, I was satisfied with one thing: I spent an afternoon lolling on a lawn and pissing vodka out of a tube from my bra. I think that's more louche than if you laid all the posh people end to end and made them drawl "Erh nohw cerm ohn, mate." We middle-class types are at home anywhere, you see.