Actually I have seen following which is bit confusing:
http://www.ukvisas.gov.uk/servlet/Front ... 4241195847
"Diplomatic Service Procedures - Entry clearance Volume 1 - General instructions (13/12/05)
Chapter 8 - How to apply for an entry clearance: the process
8.17 Validity of visa
8.17 - Visas and Leave to Enter
... There are two significant dates on the entry clearance, the "effective" date and the "expiry" date. In most cases, the "effective" date will be the date on which the entry clearance is issued. However, some applicants may not intend or be able to travel to the UK immediately following their application and ECOs have discretion to defer the "effective" date for up to three months after entry clearance has been authorised. Therefore, to prevent unecessary applications for an extension of stay, ECOs should confirm an applicant's date of travel.
For applicants in long-term categories who are subject to either a qualifying period before applying for settlement, or to a limit on their total length of stay in the category (For instance work permit holder and their dependants, UK Ancestry, au pairs and Working Holidaymakers), leave to enter should begin on the date the plan to arrive in the UK (within the three month limit). This will ensure that the maximum leave to which they are entitled is given.
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