It's hard to tell from the video but it was raining really heavily.
It was actually raining a lot heavier, but you can't really tell from the video.
It was raining a lot harder that what it seems like in the video.
「動画で伝えるの難しいんだけど、雨がすごい」は英語で It's hard to tell from the video but it was raining really heavilyになると思います。
動画で程度を伝える（動画におさめる）のが難しい時
- It was actually raining a lot heavier, but you can't really tell from the video.
- It was raining a lot harder that what it seems like in the video.
It was raining a lot harder that what it seems like in the video - 実際の雨は動画の程度よりもっとすごかった
豪雨 - torrential rain, heavy rain
イギリス 
