The best way to reach the Drysdale Building is to enter the University via de Northampton Square entrance. This looks a little messy at the moment due to ongoing works but there is an entrance from there. There is a reception desk at the entrance where you can ask for directions to Drysdale (and ELG03).
The nearest Tube station to Northampton Square is Angel on the Northern Line. From there it is a 10 minute walk to get to City, as shown in the map below:
The nearest Tube station to Northampton Square is Angel on the Northern Line. From there it is a 10 minute walk to get to City, as shown in the map below: