British Airways currently offers cheap tickets for the return flight from Zagreb to London (Heathrow). The price is even better in the opposite way, from London Heathrow to Zagreb!
Here are few dates for travel-dates in September and October this year. The cheapest return ticket on the Zagreb - London route is possible to buy for only 660 kn, while from London to Zagreb you can purchase return ticket for 556 kunas!
Zagreb - London - Zagreb
London - Zagreb - London
If you are planning a trip to London (or the opposite way), be sure to book your flight ASAP since the price is really low and will be sold out probably soon.