It's not different which is why it's so infeasible.
So, unlicensed are you or are you not allowed to fly within 50M of items / people not under your control?
Not allowed if you have a camera fitted. Remember pretty much all the rules only apply if there's a camera fitted. No camera and you could fly into your neighbour's garden, almost land, and you'd be fine.
When licensed, you can fly within 50M of items / people not under your control?
No, not without explicit permission. When we receive our permission for aerial work through it'll have a few exemptions, one will be that. The others include night flying, flying in London, etc.
What constitutes a congested area?
I don't think there's a specific definition of that, but in any case that's 150m as well. You're basically talking an A road, motorway, generally busy area. I personally wouldn't count buildings or where people are inside a factor to this. Common sense applies really despite that not really existing in the CAA.
I was just thinking, with the 50M rule, technically I can't fly in my back garden because I don't have control over my neighbour's house.
Yep, technically, which is why it's stupid. But again that's only with a camera attached. No camera, you're fine, so their argument would be "why do you want to fly with a camera in your back garden" and "if you do, go fly somewhere else". Stupid I know.
We've probably broken quite a few rules if you're following it letter-for-letter because most are impossible; 50m is a huge distance to be away from anyone or anything when doing commercial work (there ain't a lot of work going round in the middle of nowhere).