"no your average middle class London Victorian terrace house so not big at all. Most of her peers, at least the ones who threw these parties, live in similar houses so I’m not sure."
Do you know the parents of any of your DD's peers who have hosted a party for 50 16-year-olds in a London Victorian terrace house? If so, I think you should have a chat with them, to see how things worked out.
I'm not sure how large a London Victorian terrace house actually is (e.g. a long narrow hallway, leading to two fairly small reception rooms, one behind the other, versus, two large reception rooms, either side of a massive entrance hall, with a family room and a large kitchen dining/kitchen behind those
)?
If the former, you are mad, your house and garden will be wrecked (possibly your neighbour's gardens as well). If the latter, you are still mad, your house and garden will still be wrecked (possibly your neighbour's gardens as well).
It sounds like you really want your DD to have the party of her dreams (not to mention being the coolest, most laid back mum ever), but unless you are planning a very stiff couple of hours with strong parental supervision, then I think you are being unrealistic in thinking you can accommodate that size of party.