My UK visit was approved yesterday. I am planning on visiting the UK in June and right after Spain. Do I provide the Spanish embassy with my planned itinerary for the UK as part of of my application?
No, you're only required to submit your itinerary for Spain (such as hotel bookings), along with proof of how you're entering/exiting (i.e. a ticket on either Eurotunnel or a ferry).
It will make sense to the visa officers seeing as you already have a UK visa