Thanks for your post. I have the same problem. With the current system you cannot specify the coordinates exactly in the map – but you can check the coordinates after download in the files.
It would be useful if either a) the coordinates of the download bounding-box (or point) are displayed in the map browser, and/or b) the bounding box or point coordinates could be set in text input fields. I hope the webteam reads this and can do something about it.
As a workaround with the current system, I have zoomed in to see the more details in the background map and compared to google maps where you can get the coordinates. For larger areas, I have used trial-an error with one of the HydroGFD ECVs until I have a selected bounding box covering my area of interest. Then I keep this webpage and selected area and change variable to download until I have all data (for the GCMs I can be quite painful to go through this process if your area is large.