If you want a cheap flight from Sarajevo to London, you've got zero airlines to choose from.
London City Airport is just one of the airports in London. If you're not set on a specific airport, try changing your search from London City Airport to London to see a broader range of flights – they might be cheaper.
During this peak season, hotel and holiday prices often increase too, so it's best to book early. Or you could keep your fingers crossed and hold out for a last-minute deal.