The main one which I'm going to jump from is Pontcysyllte Aqueduct it's 130ft high and crosses over the River Dee and beautiful landscapes. You can easily access this place as there is a footpath you can walk down next to where the small boats cross. I've been here at night time and it is extremely quiet. I went at 2am and didn't see a single person. There isn't high fences it's easy to climb over. And there is no reported survivors who have jumped.
Here's one article on the viaduct related to suicide
https://www.dailymail.co.uk/news/ar...-hours-breaking-girlfriend-inquest-hears.html
A few more high places are in the north west are:
Gathurst viaduct (shevington) - 75ft
Thelwall viaduct (Warrington) - 95ft
Thornton viaduct (Bradford) - 120ft
Sankey valley viaduct (St Helen's) - 80ft
There is loads of viaducts everywhere. The reason why I chose viaducts is because usually they are easily accessible and are often 70ft+ high which is guaranteed instant death. Most of them have really nice views too so it's a win win situation.