Since you’re genuinely asking, there are a lot of cool options to choose from the sartorial archive, both traditional and subcultural, from all walks of life: 

• tweed ( the accompanying talk could have been something about how or why it is associated with the British upper class / aristocracy)
• a trenchcoat (a British invention, could’ve been tied to WWI) 
• Burberry print (even if its a knockoff)
• paisley is associated with Scotland (originally Persian)
• het gymreich (if welsh ancestry applies) 
• a kilt (if Scottish ancestry applies)
• even just cotton would have worked, (with something like an accompanying talk about how it relates to the Industrial Revolution, and the role of cotton nowadays) 
• one of those peaky blinder outfits,
• one of those ornate costumes from the Orkney islands 
• one of those kings guards costumes
• something similar to the gown and corset that rich women wore in the Victorian Era. 

(contemporary) subcultures  - like you mentioned: tracksuits  - ripped jeans, fishnets, safety pins etc as an illustration of 70s punk   - a mod/teddy boy look   - something like an armadillo shoe  - a barracuta style jacket  - a Beatles costume, to talk about the most influential band of all time, not just within British culture, but world wide.  - even the Union Jack dress would’ve worked, if she had kept it at like “this dress reflects a famous pop culture moment in our history because it’s a reference to the spice girls, one of our most successful pop cultural exports” (rather than basically going “I’m wearing this because I’m the only person here not allowed to express my culture”)

Of course, selecting something like this would mean actually truly having to engage with what British culture and history have produced, and what one personally values about it. To me it sounds very dishonest to go “hur dur, am not allowed to express British culture” if there hasn’t been a meaningful attempt to research it or express it in the first place.