This British trend of naming children after George R.R. Martin fictional characters began in the noughties, when the books were released. It started in 20o1, with 3 boys being named Bran, the second Stark son, and in 2012, 5 boys were named Theon, after the heir to the House Greyjoy.
The largest peak happened when the television series first aired in 2011. In 2014, 18 Theons, 17 Tyrions, four Brans and four Sandors were recorded.