You should aim to enter get your passport stamped within 2 months of receiving approval for the Highly Skilled Migrant Programme and ideally to enter the country within one or two months of this point. You should note that it is at the discretion of the embassy or consulate to grant the full 12 months that the Home Office approve, although it is extremely rare for them not to grant the full 12 months. This initial period commences upon your passport being stamped and NOT when the HSMP is approved. Towards the end of the initial 12 month period you can apply for a 3 year extension, provided you can demonstrate that you are able to support yourself and ideally have found gainful employment. HSMP, as with a Work Permit, counts towards UK Residency.
Hope that helps,
Regards,
Dean Marshall
Director
SkillClear
newcomer link: http://www.skillclear.co.uk/ [nonactive]