A 4 to 6-night trip to Barcelona in September 2026 for 2 adults from London costs $594–$841 in verified flight+stay combinations, based on 5 real date options Valora priced against live supplier data, verified as of Jun 29, 2026. The best-value option departs Sep 1 and returns Sep 5, on Vueling, staying at Sant Jordi Hostels Rock Palace, for $623 total — flight, stay and estimated transfers for those exact dates and 2 adults, never a per-night rate scaled up. The full Edit below compares every option and adds the weather, sea temperature, crowds and local knowledge for those dates, with a source link on every claim.