Whether the power lines were there before you or not is irrelevant. You can still make a claim! This is because a claim for power line or pylon compensation is based on the fact that those power lines or pylons are currently over or on your property with no permanent legal right for them to be there.
You are essentially claiming that your property would be worth more without the power lines or pylon being present, than it currently is. This argument is accepted in law, irrespective of whether or not you actually paid a reduced price for the property and it doesn’t matter that the power lines or pylon were there before you purchased your property.
The electricity company have to compensate someone for the devaluing effect that their power lines and pylons have on the land and so it stands to reason that it should be whoever happens to own the property at the time the claim is made.
Remember, compensation will only be paid once in respect of any given property. If you do not claim, a future owner probably will, so why miss out?