Best place for work experience

I am a young(mid-teens) Student wondering if anybody has any recommendations for somewhere in London for buy-side work experience(or anywhere you thing would be good)

Looking for somewhere interesting and not of a scale to be intimidating. 

I would hope to be of some use but am away that sometimes with this type of thing the firm plows rather more resources in that can really expect to get out.  I am of course happy doing about anything around the office but would like to learn something.

Any places you would think are worth writing to?

Thank you for any replies