Holiday booking - moved goalposts

My wife booked a package holiday (flights, hotel and transfers) with Destination2, a travel agent.  The booking was on the basis that we paid a deposit, and the remaining payment was due 12 weeks before we go – the trip is not until March 2027, so payment in December
 
5 days later the agent called and said the airline has “ticketed” the flights and she needs payment of circa £6k immediately to keep the flights.  The outward flight and the return flight are different airlines, but she says part of the same group.  Obviously this is way more than we expected up front, so my wife asked to cancel the holiday and return the deposit, desitnation2 have said the deposit is non- refundable.  The deposit is £1400, so I’d rather not lose it.  
 
They have offered less favourable flights (And £500 more!) which are on the 12 week before departure payment, which my wife has had to take, but I am not happy.