The e-commerce firm, which helps sell retail brands including Lookfantastic and skincare group ESPA, gained 25% in its first day of trading.
The blue-chip FTSE 100 fell 0.4%, with strength in the pound weighing on its export-heavy constituents. The head of the European Commission said the chances of reaching a trade deal were fading by the day as the UK government pushes ahead with moves that would breach their divorce treaty.
The threat of a no-deal Brexit adds another layer of uncertainty to the British economy, which is still reeling from the shock of the coronavirus outbreak.
"A new word has been invented - Le Brovid - to describe the simultaneous fears of a no deal Brexit and the inability to get the UK COVID-19 reproduction rate below one," strategists at Jefferies wrote in a note.
The mid-cap FTSE 250 fell 0.1%, with losses mitigated by continued gains in insurers after a court ruling over COVID-19 payouts was seen as favourable.
Investors are now looking to the Bank of England's policy meeting on Thursday for signs of more stimulus, a day after the U.S. Federal Reserve is expected to wrap up its meeting with a renewed pledge to keep interest rates low for long.
British homebuilder Redrow fell 1.8% after reporting a plunge in annual profit, while promising to renew dividend payouts next year as it halts investments in London and focuses on its Heritage Collection homes.
Construction firm Galliford Try Holdings rose 3.5% after it said it expects to return to profitability in fiscal 2021 as site activity resumes and productivity nears normal levels.
(Reporting by Shashank Nayar in Bengaluru; Editing by Uttaresh.V and Mark Potter)
By Shashank Nayar and Ambar Warrick