As a principle, I would recommend using an agent - they supply all this kind of thing and it costs about GBP 50 to get the company registered including a standard set of paperwork - see this question for some suggestions. My view is that unless this is your core competency, your time and effort could be better spent that on the fine details of company formation.
Having said that, from 1 October 2009, there was a change in UK company law which meant that a separate Memorandum of Association was no longer required - the official registration form you submit to create the company is effectively the memorandum of association - it states who the intended shareholders are, that they wish to form a company and that they agree to be shareholders of it.
Separately, you will need a set of 'Articles of Association' or 'Articles' for short - this is the document that states what activities your business is permitted to conduct (i.e. it's own house rules, if you like). A standard set of articles usually let's you do anything that's legal; however, it is unlikely to matter much until you come to sell your company (for many millions :-) of course).
A good book to understand what's involved in starting and running a UK limited company is "Running a Limited Company" by David Impey & Nick Montague (although make sure to get the latest edition).
http://www.legislation.gov.uk/uksi/2008/3014/pdfs/uksi_20083014_en.pdf Explanatory Note (p6) (note the "Schedules" are included in the pdf):
The Companies Act 2006 (c.46) makes changes to the form of a company’s constitutional
documents. The Companies Act 1985 (c.6) and the Companies (Northern Ireland) Order 1986 (S.I.
1986/1032 (N.I. 6)) both require a company to include a substantial amount of information in its
memorandum of association and allow a company’s constitutional rules to be divided between its
memorandum and articles of association. Under the Companies Act 2006 all the constitutional
rules will be contained in the articles of association; the memorandum of association will be a
much shorter document.
Regulation 2 prescribes the form of the memorandum of association required under the Companies
Act 2006. The forms in Schedules 1 and 2 serve the limited purpose of providing evidence of the
intention of each subscriber to form a company and become a member of that company and, in the
case of a company that is to have a share capital on formation, to take at least one share.