Schengen Question

My daughter is studying abroad over the summer and does not require a Visa. She entered the Schengen area on May 21st and will be leaving it on August 19th, which is 91 days (our bad – her dad and I are meeting her in Italy and traveling to Greece with her and I did not understand the 90-day rule). She is planning to travel to England/Scotland for a few days while in Italy. Can we subtract those days from the original 91 days? Thanks much.