At the very least an hour. Personally I'd say 90 minutes. For all 4 of your flights.
If JFK is like LAX Virgin Atlantic Flights start boarding a good 30 minutes for departure.
You never know what security lines will be like. Could breeze right through. Could take forever.
Given the popularity of travel to London I'd never assume even an 8:05am flight wouldn't be full.
So get up early, arrive early and don't stress. It's not like major airports are vapid wastelands where you can't find something to do.
If you've never flown Virgin before you are in for a pleasant surprise. Love this airline for comfort, the amenities
( a small bottle of water will be on your seat), excellent choice of films, TV shows and music. Well trained, attentive and polite staff. And for airplane food, pretty darn good.
Enjoy London.