Sadly I'm just in London for a week
in that case, probably your best bet would be to make contact with some helicopter flight schools around the outskirts of London.
I think the guys at HQ Aviation Denham used to allow people to look around their aircraft, and are certainly passionate and knowledgeable about what they do, but probably worth a call first to avoid a wasted trip just in case.
There's also HeliAir at Denham, who you could perhaps approach, plus I think there are a couple of helicopter filming companies there too.
I'm sure there are other companies at different airfields around the outskirts of London as well, but again probably worth a call first, as some places are likely to have a policy of absolutely no access to their hangars for non-employees/customers.
You'd probably get bigger aircraft at Battersea...but, I suspect hefty parking fees once the aircraft are on the ground more than 15 minutes might limit your chances of actually approaching aircraft/pilots.
Hope this helps!