Basic tax point and Actual tax point

Then the the actual tax point will be the basic tax point. The rule is that the basic tax point will apply unless as you say an invoice is issued within following 14 days, or an invoice is issued or cash is received before the basic tax point.

If you mean the qualifying time period for making the reinvestment, rather than the date by which the election must be made, it is a 4 year period running from 1 year before the disposal date to 3 years after the disposal date.
If for example a business property was sold on 1 January 2015 the gain arising may be rolled over so long as a qualifying asset was purchased between 1 January 2014 and 31 December 2017.
If you are confused by the idea of reinvesting BEFORE the disposal date, then think from a practical point of view a business would wish to acquire a replacement property before the disposal date of the old one to avoid any break in trading that would otherwise occur