In reply to The Lemming:
Not sure what camera you have (unless it's the GH4 as per your gallery in which case I don't have a clue what's available)
But the Canon 400mm f/5.6 is great value and relatively lightweight.
Both Sigma and Tamrom do 150-600mm lenses which get well reviewed.
Budget is quite a major factor though. Sadly the light in the UK is usually less than fantastic, so a fast lens can be a real bonus. But fast + long is never cheap.
Good high-ISO can mitigate this to an extent, but then you lose lovely shallow DOF.
I used to have a 300mm on a crop body and often found this too short for nervous wildlife or small birds. Bushcraft/fieldskills can improve this, but I'd say you really need 300mm as a minimum.
I rented a 200mm lens for my full-frame A7R2 when I went to Canada as there aren't many longer options, and really struggled to fill the frame, even with really tame/nonchalant elk (Fortunately 42mp offers a lot of crop potential!).