Setting - I have heard it starts in Y2 and it's only for English and Maths. But if you want to know for sure, better ask the registrar.
Of course you can't know how academic your child will be at 2.5 (or even at 4.5 for that matter!).
But in your case, that shouldn't matter.
If you take the NP nursery offer, you get a free option of automatic entry into NP reception (maybe not entirely "free". as you give up your current nursery
). Then you continue to look at other schools for Reception entry, go for assessments, etc - you may get something you like better, or NP may do enough to convince you to stay.
The reality of getting into a London pre-prep via assessment is that it's pretty much down to luck (I mean, what can you really "assess" at the age of 3??). So having a certain place at NP is very attractive.
Re. the comment that NP is very academic - I don't think that's true.
It's more academic than other schools, but their yearly intake is approx. 80 children, and there is just no way that all those 80 children in a year group are all academic. My friend's son is not academic at all, but he loves the school and is very popular.
From my research last year (when we were choosing a school), from approx. Y2 onwards in most pre-preps there will be 1-2 children per year group who will not "fit in" and they will move on. I have heard of children moving from Eaton House to NP, from Thomas's Clapham to Broomwood Hall, from Broomwood to Streatham & Clapham High (there was a thread about this on NVN last year, but cannot find it). The majority of families will be happy to stay at their current school. But cannot reasonably predict what your child will be like in 4 years' time, so you can only make a decision based on your current circumstances/thinking (and be prepared to move if you do end up in the 1% of cases where the school you have chosen is not a good fit).
Re Broomwood Hall Garrards Road - cannot see how this would work for you, as it's miles away from NP (which you say is close to your house). Broomwood Hall is also very different from NP, so if you like one you probably won't like the other (small, more traditional for BH vs big with lots going on and modern facilities for NP). Generally families who like NP also like Thomas's (also big, coed, with lots going on). And before someone gets offended ...I DO NOT, in any way, mean to suggest BH is not good - just that different schools appeal to different families.