I want to understand where to look or how to research who the main or
key shareholders are in any given UK PLC.

e.g. 
(1) who are the institutional investors and what % do they hold. 
(2) What are the % of each board member

is there a specific place or places where I can go for this information?

Dear andybromley-ga,

Shareholders for Stock Exchange listed companies are required under
the Companies Act to make disclosure of their shareholdings when they
reach a certain level in terms of percentage of the total shares
issued by the company. Directors are required to make to make
disclosures at all times. These disclosures are going to be your best,
most easily obtained source of information. Unlisted companies have
their shareholder details filed at Companies House.

This Motley Fool web site article explains in simple terms the
requirements of the Act.
http://www.fool.co.uk/news/Comment/2005/c050119a.htm?source=EDSP

It does comment that "Thus observers monitoring the goings-on within a
company's share register sometimes have to plough through previous
announcements and/or refer to the company's latest annual report
(which ought to list shareholders with 'notifiable interests') to
discover what the large players are up to."

The Corporate Watch web site also gives some guidance on finding shareholders.
http://www.corporatewatch.org.uk/publications/diy_research_2002_4.htm#shareholders

There are a number of resources you can use to research shareholdings.
First, each company will state in their Annual Report the largest
shareholders as of a certain date. You can normally obtain these from
their web site or by post from the Company Secretary.

Next, taking ICI as an example, you will find listed on UK-Wire all
the disclosures required under the Act for directors and others. Click
on each announcement and you will see their names and shareholding.
http://uk-wire.com/cgi-bin/index?search_type=3&words=ici&go.x=11&go.y=9&go=Go

Hemscott.com provide a very good service which will probably meet all
your requirements as they summarise the current main shareholders. The
companies they provide information on is listed here.
http://www.hemscott.com/equities/atoz.htm

This is the link for ICI and here you will see their main shareholders.
http://www.hemscott.com/equities/company/share/shr00365.stm

Click the link on the left to see the directors? shareholdings. You
will note these are not summarised.
Motley Fool does provide that service. This is the link for ICI
directors? shareholdings.
http://fool.digitallook.com/?action=dealings&ticker=ici

As mentioned above, Companies House provides shareholder information
but you have to purchase each of the company records to research them.
This is the link for Companies House.
http://www.companieshouse.gov.uk/toolsToHelp/findCompanyInfo.shtml


I hope this answers your question. If it does not, or the answer is
unclear, then please ask for clarification of this research before
rating the answer. I shall respond to the clarification request as
soon as I receive it.
Thank you
answerfinder

Search strategy
Personal knowledge and: uk directors shareholding disclosure
://www.google.co.uk/search?hl=en&q=uk+directors+shareholding+disclosure&btnG=Search&meta=

exactly as requested