This cache is placed in a peaceful and quiet part of Mill Hill, near where Mill Hill borders Hendon. There are lots of nice walks around a green area. Here's a fun fact; the modern microwave was invented within a few hundred metres of where this cache is before the area became a RAF barracks for many years before becoming a public area.
Like my other cache over in Totteridge, this is another coordinate calculation one but hopefully won't be too difficult. 
I've made it a bit convoluted and your mission, should you choose to accept it, is to flap away the mists of mystery with your butterfly wings of puzzle solving!
So, to get the co-ordinates of where the cache actually is, solve a few quick puzzles! Once you have the co-ordinates, have a look around that area but see the hint if you would like more specific instructions once you are there.
51.6XXX3X, -0.20PPPP
Note that this is decimalised rather than with degrees or otherwise but inputting it should work fine.
From the starting point, look for a bus stop outside Pirate Park. This bus stop only serves one route which is a 3 digit number. Take this number and multiply it by the number of words that make up the name of the largest species of butterfly in the world, (ignoring the word 'butterfly' if it should state it with the word 'butterfly'). This number should give you XXXX. Be careful! The XXXX is split by a number 3 in the above coordinates, so if your answer was 5678, the first part of the coordinate would be 51.656738
At the entrance of the Pirate Park a little further along, there is a parking enforcement sign. Take the number of hours a day it is enforced and multiply this number by the number of wings a butterfly has multiplied by the number of legs a butterfly has muliplied the number of words that make up the name of the smallest species of butterfly in the world. (Do not include the word 'butterfly' when counting the number of words for this last bit). Finally, add 8146 and your answer will be PPPP.
To find the cache, look for where green no longer meets green. (The location is not at all far from where you began).