I am currently in the US under a J1 visa programme, and will leave in July 2016 however I would like to come back to travel in the following month of September 2016 on an ESTA? Would this be okay? Is there any time preference for staying in the UK before returning to the US for leisure purposes? Also if I was to then return in December for a Christmas holiday vacation under the same ESTA visa would this be okay? Understanding that I cannot stay longer than 90 days in total.
What you are planning to do complies with the rules. There is no "minimum waiting time" until you can re-enter under the visa waiver program after an expired J-1 visa (and after your DS-2019 expired), provided that your reason for visiting the US is OK with the VWP rules.
Yet, it is very much advisable to bring all documentation that you can provide to show that your are not trying to circumvent the rules, such as, e.g., hotel bookings, conference invitations (if you are travelling to a conference), etc., for your VWP trip. You will need to convince the border agent that you are not doing anything wrong.
Note that if you want to do holiday travels within the US immediately after the end of your J-1 program, you may be able to use the 30-days grace period to do so.