Giant Defy 2 Bottom Bracket
Ultegra is still hollowtech II. It seems like the Defy 2 in the UK has a pressfit bottom bracket for hollowtech II cranks, while in the US it has an usual threaded hollowtech II bottom bracket. In any case, you need a hollowtech II compatible bottom bracket, which you already have (Tiagra is Hollowtech II only).

Just remove the current crankset and put in the ultegra crankset. 

Aside: This guide from BikeRadar summarizes bottom bracket standards.