What to Wear? What to Wear? (Wrings hands)

As I I did not have more important things to worry about, I am stressed over what to wear to Polish Pretend Son's wedding 2.5 months from now.

My stress began when I was in Toronto and I dragged my friend Trish (who has a car) to various fabric shops. We ended up in Fabricland before I realised I didn't like any of the patterns except for the mock-mediaeval gown. It was clearly a mistake for women to move on from 14th century fashion. However, it would also be mistake to go swanning into the Basilica of Saint Mirosława the Miserable in a mock-mediaeval gown. It wasn't even authentic; there was something wrong about the fastening, as Trish (expert) pointed out. 

We left before I could wig out over the pattern books, and that night I had a very bad dream in which Trish was in love with a Stalin-Putin hybrid, but he was in love with me. It was quite an uncomfortable situation, and I was very relieved when I woke up. 

A blogpost has suggested that one should not wear to Polish weddings what one wears to English ones (e.g. lovely hats) but instead pick something one could wear to a high-end disco. 

Unfortunately I have not the slightest idea what one wears to a high-end disco.  The last time I went out dancing it was to Balkanarama and I dressed like this: 

Excitingly, although I was with two young female colleagues of my husband, I was the first one to get chatted up. This may be because young British men have a soft spot for the over-35 set, or maybe it was a sneaky trick to lower my companions' confidence so they would talk to simply anyone afterwards. I suspect one day this will stop happening and I will have to get ego-boosts from elsewhere. On the other hand, elderly widowers would approach my grandmother when she was visiting the cemetery and invite her to the adjacent McDonald's. It's not over till it's over. 

I came across this "high-end disco" advice very recently. Until then I was gazing at splendid English gowns I cannot afford. (See here and here.) As I always say, British shops have splendid dresses for women, so it is a shame British women do not wear them more often. Although I keep staring at the website for my go-to (and bought this little bargain on my lunch break), I cannot find a dress that says "I am the perfect dress for traditional June weddings in Poland!" 

So naturally I am asking for advice. I like long and floaty with sleeves, the budget goes no higher than £200, and I prefer to buy British.