It would be easy if you just had to create one site and then clone it 99 times. But if you clone your site, search engines are going to put you in the black list and that means no traffic and therefore no clics and no ads.
Reaching the top is much harder than staying at the top, in my opinion. Since you pretend to do this in the long term, I suspect that you want to stay at the top (whatever your goal is) and that means quality. You can't keep the quality of a hundred sites by yourself (you can if you have a good and a large team). You can do it for one, two or maybe even five but not much more than that and depending on your niche and goals.
To do as "little" as £10 per day and site, you'll need a minimum of 1000 visitors per day (being conservative). Of course, it doesn't matter if they're new visitors or not, simply that they clic the ad. That means 1,000 x 30 days = 30,000 visitors/month. Per site.
You can reach there, for sure furthermore if the niche is not being exploited right now. But you're understimating certain things such as:
- the amount of work it takes
- that you're going to be tired, and fed up, and you will not have always the same energy every day
- the amount of disappointment you will feel when you don't reach the figures you expected when you planned
- the amount of times that you will want to quit
- the amount of traffic varies... one day you're on the top and the next you have a hundred sites above you
- it is continous work. You don't do it and then leave it and get residual income. You still need to keep on going (although, it is true, that not as hard as before)
- If you don't love what you do, you're going to be moody pretty soon.
You don't choose a niche only because it looks like you can make good money. You choose it because you love the subject AND you can make good money.
And then, you have a plan B in case that plan A doesn't work.
Your plan A is to build a 100 sites and get some cash from ads... what's plan B?
In my opinion, it is easier to sell something (let's say an eBook) and earn £100 per day than building 100 sites and earn £10 per day from ads.
But that's just me. I could be completely wrong.