If you assume that Euphoria opens on Friday 18th June, you can get a BA flight from Gatwick on the Friday night at 8pm which lands in Ibiza at 11pm. Should be enough time to get you to San An.
You can then get a BA flight on the Saturday night back to Gatwick leaving at midnight on Saturday night/Sunday morning, arriving in Gatwick at 1.10am.
Total cost is around GBP 175
You probably will be able to get similar flights to this with Easyjet from Stansted and will probably be a little cheaper but schedules not out yet.