Whether you can start work straight away will depend upon whether the permission is subject to conditions, for example, requiring you to submit for approval details of a specified aspect of the development which was not fully described in the application, if it is then these must be dealt with before the development can begin.
Furthermore, if outline permission has been granted, you will need to submit a further application for approval of anything that was not covered by the outline application (known as "reserved matters") before starting work. This must be done within three years of the grant of outline permission.
Otherwise as long as there are no conditions requiring further details to be submitted for approval prior to work commencing then you are able to start work straight away (subject to other legislation and requirements such as building regulations approval).
It should be noted that planning conditions are applied to almost every grant of planning permission and are attached to ensure that the development is acceptable in planning terms. Conditions are important and must be read and understood prior to starting your development as they can limit and control the way in which the planning permission may be implemented. Any planning conditions will be listed on your decision notice.