Oh, and there's BUNAC:
http://www.bunac.org/usa/interninbritain/Providing you meet their eligibility requirements:
Be aged 18 or over
Currently be studying in the USA or be a US citizen studying overseas
Currently be a full-time degree level student, or a graduate intending to begin the internship no more than 12 months beyond the end of study
Hold an acceptable internship offer in the UK for up to 6 months
Have sufficient support funds
Unfortunately, there's no 'one year visit' visa, I'm afraid. You could apply for a visitor visa. As a general visitor, you must:
1. intend to visit the UK for no more than six months;
2. intend to leave the UK at the end of your visit, and
3. have enough money to support yourself and live in the UK without working or needing any help from public funds.
See
http://www.ukvisas.gov.uk/en/howtoapply/infs/inf2visitors#21460086 If you want more details, post your queries in the applicable board - happy researching!