I got my Family Permit on 14th January. In my application I entered 15th Feb as my 'Intented Arrival Date'. Is it ok if I arrive at 20th Feb as my Family permit is valid for 6 months. Or should I reach UK on 15th Feb?
Plans change. There will be no trouble if you arrive on the 20th of February or indeed on any other day during the period of validity.
As always, there could be trouble if any change in plans leads a country's officials to suspect fraud or other misrepresentation. Delaying a trip by five days is extremely unlikely to arouse such a suspicion. If such a suspicion does arise, a reasonable explanation for the delay will almost certainly dispel it.