Flying to incredible Nepal off peak season is possible on board of Star Alliance carrier Turkish Airlines. Flights from London to Kathmandu are available for £296 and Madrid from €346! To reach the lowest fare available you would have to accept long layover in Istanbul on outbound or inbound flight. Turkish Airlines belongs to the carriers offering free hotel during layover at their main hub – check out this article if interested in further details.
If interested in this Turkish Airlines deal you will find the cheapest flights from the end of Aprill till end of September. (Based on date of your outbound flight from Europe). including flights in July and August. (Althought it is not recommended to visit Nepal during our summer holidays as it is monsoon season with heavy rainfalls). You can easily compare all available travel options directly at website of Turkish Airlines.
Booking sample of cheap flights to Incredible Nepal from London £296 or Madrid €346! In this case you would depart from Spain to Kathmandu on 8th of May. You would return from Nepal to Europe on 22nd of May 2017.