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I really don't know. Maybe call the admissions department of the school you are interested in. My understanding was that they allowed non-resident students, they just charge them more, ala:
http://www.kbcc.cuny.edu/admininfo/Tuition.htm

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Your best bet is to call.
http://www.lagcc.cuny.edu/contact_us.aspx
(718) 482-5000 for LaGuardia admissions. You can use the webform at
https://www.lagcc.cuny.edu/admission/callcenter/requestinformation.aspx
which is like email, but if they are anything like the other colleges I contacted for my ex they will postal mail you stuff and it will take weeks and weeks and weeks. But you could try, maybe they will email you back.

Not sure what you are after but you can take classes at places like the International Center of Photography - www.icp.org
You can take whatever classes you want but it isn't that cheap although being a Londoner you should be able to enjoy the weak US dollar