The route of London to Lahore is very long so the flight charges mostly depend on the route and also the weather situation or the popularity of the airline. It is not such an easy task to find out which flight gives the cheapest rate to travel from London to Lahore. You have to visit several travel agencies and check out the price each one of them are offering.
Only then you can decide which one suits best for you to travel. It is very important to take help in this matter from some travel agent. If you know any agent then its best, he/ she may guide you in a very good way and will also help you in getting the lowest rate flight with facilities.
However, you can also do an internet search on this matter. But also by this way you may not get reliable results. The route is too long therefore the rates of flights will definitely vary, in an approximate you will be able to get flights that start from $ 250 and can go to an undefined number. So it also depends on your luck sometimes.
In any way do not get fooled, as some travel agent may give you a ticket that is on a chance, which means that if any registered passenger leaves his/ her seat or in case there is a spare seat left in the flight you will be registered to it. While, if no such condition gets fulfill your money will be lost without getting any seat.