London → Barcelona

The single biggest driver of price here is the calendar. School holiday blocks, long bank-holiday weekends and the broader summer travel season pull huge numbers of travellers into the same few weeks, and that is when cheap seats disappear first. If your dates are locked to one of those periods, the sensible move is to start looking well ahead and to accept a decent fare rather than hold out for a bargain that may never appear.

If your dates are open, the opposite logic applies: use the flexibility. Compare a run of departure days on this page instead of a single date, look at the shoulder weeks either side of the obvious holiday period, and check whether a Tuesday or Wednesday departure prices differently from the Friday most people want. Outbound and return legs are worth pricing separately too, since the cheapest combination is often not the most obvious one.

Comparison is the whole game on a route with this many operators. A single airline's website can only tell you what that airline charges, which is why the same search across several carriers so often turns up a materially lower number. Once you have a shortlist, read the fare conditions before deciding: cabin bag rules, checked baggage and seat fees vary enough between airlines that the cheapest advertised price is not always the cheapest trip.

Timing helps too. When you are not in a hurry, follow the route for several days and note the range fares move within, so you can recognise a genuinely good price when one appears. Pingsme is free to use and updates at least daily, so checking back before you commit costs you nothing but a minute.