sorry luxie, I hope you don't mind me helping you trying to figure out the visa stuff. I am in the middle of getting my work permit visa and entry clearance so I am very familiar with the visa website and I just like to point out my understanding of things.
No you don't need a visa according to the site. Try this site
http://www.ukvisas.gov.uk/servlet/Front?pagename=OpenMarket/Xcelerate/ShowPage&c=Page&cid=1006977149962Take note of this message:
"Change to visa requirements from 13 November 2005
From 13 November 2005 all those who are not British or EEA nationals will need a visa or entry clearance for all stays in the UK over six months."
If you are American, currently in the US and wanting to study under 6 months this is the response you get:
http://www.ukvisas.gov.uk/servlet/UKVisasDoINeedAVisa?url=%2Fservlet%2FFront%3Fpagename%3DOpenMarket%2FXcelerate%2FShowPage%26c%3DPage%26cid%3D1006977149962&purpose=Student+six+months+or+less&nationality=United+States&location=United+States"You asked if a national of United States needs a visa to come to the UK as a student for six months or less.
No, you do not need a visa.
On arrival in the UK you must satisfy an immigration officer that you qualify for entry to the United Kingdom as a student for six months or less under the Immigration Rules.
Students
Please note that if you are allowed to enter the UK as a student for up to six months, you will not be allowed to extend your stay unless you arrived with a student or prospective student visa, or are studying on a course at degree level or higher.
Please read Guidance - Students for further information. Your sponsor, if you have one, may want to read Guidance - Sponsors.
N.B. Even though you do not need a visa, if you have previously been refused a visa for, or entry to, the UK you may want to apply for one to find out if you qualify for entry before you travel. Please contact Chicago, Los Angeles, or New York for further information."
The guidance notes has TONS of info to help answer your questions including about you working.
http://www.ukvisas.gov.uk/servlet/Front?pagename=OpenMarket/Xcelerate/ShowPage&c=Page&cid=1018721067373"Can I work?
You can take part-time or holiday work, but you must not:
work for more than 20 hours a week during term time unless your placement is part of your studies, has been agreed with your educational institution and leads to a degree or qualification awarded by a nationally recognised examining body
do business, be self-employed or provide services as a professional sportsperson or entertainer, or
work full-time in a permanent job.
If you are coming to the UK as a student for six months or less, you must ask the Entry Clearance Officer for permission to work."
Here is the part about extending your stay"
"If you do not need an entry clearance, you will have to satisfy the Immigration Officer that you qualify for entry when you arrive in the UK. They will then give you permission to stay in the UK for six months. You will not be allowed to extend your stay in the UK as a student unless you arrived with a student or prospective student visa, or you are studying on a course at degree level or higher.
To extend your stay you will need to apply for a residence permit at the Immigration and Nationality Directorate. (Contact details are under 'More advice and information' at the end of this guidance.) They will charge you a fee for this.
If you have any doubts about whether you qualify for entry, you should apply for a visa before you travel to the UK. "
but it doesn't clarify if you need to change it to a visitor's visa for the time beyond the student visa.
