Oh sorry, I didn't explain that very well. BST is GMT that accounts for daylight savings time. So since we account for daylight savings time in the US, you are already one hour ahead of your normal time difference with GMT. So you retain that -8 time difference in the summer with BST.
Note to people in other regions (Australia, Thailand, etc.): It's important to check the map I posted to see if your region uses daylight savings time. If this is the case, subtract one hour from the time listed above and just use GMT. (5:00PM - 8:00PM GMT)