As you might expect with it being the capital of Pakistan, there are a number of airlines offering direct flights to Islamabad. Although the cheapest flights to Islamabad will usually be indirect, if time is of the essence you can fly direct from Heathrow, Birmingham, Manchester and Leeds Bradford airports.
However, if you are looking for a cheap ticket to Islamabad, then you might want to consider using one of the many quality airlines serving the city via intermediate points in the Middle East and Europe. Airlines offering cheap flights to Islamabad include Gulf Air, Etihad Airways, Oman Air, Kuwait Airways, Qatar Airways and Pakistan International Airlines.
Tickets to Islamabad vary in price depending on several factors including the time of year, the class you wish to travel in and whether you wish to travel directly or are prepared to (or wish to) make a stop at an intermediate point. In general, if you are flexible with your dates (even a day or so either side can make a big difference), if you do not require a changeable ticket and are willing to book in advance then you will get the cheapest flights to Islamabad. If you are looking for premium cabin tickets to Islamabad, prices will naturally be higher but you will benefit from the advantage of having more flexibility with your ticket.