DS just got back from undergraduate admissions interview at Oxford. In preparation for the trip, we bought a UK SIM card on Ebay. The brand we bought, came with 10 Pounds credit on it, and we were able to activate here in the US before he left. In that way, we already knew what his UK telephone number would be in advance. We bought on the Three network; they have a 30 lb/month rate which gives unlimited phone/text/data with no contract required. Top Ups can be bought at any tobacco/convenience store–like our Seven-Eleven or equivalent.
He was able to Facetime us using his Apple device using the college’s free Wifi without using any data allowance.
One additional issue, suggested by boomting’s first post on this thread–is tranferring money to your student may be easier if you deposit money into a bank that has both US and UK operations. HSBC may fill that bill.