that cup on the drive side is called a "fixed cup" and it not intedned to be removed for routine maintenance. since you are having the frame sandblasted, you may want a shop to remove it for you, they are in pretty tight.

as for sandblasting, they may be able to do the inside of the headtube but not the BB shell as it will ruween the threads.

do a search for Oxicillac Acid on here. lots of guys use it to clean out old frames.

I am all for keeping it original. I try and keep my older bikes as original as I can with the exception of stem and bars. sometimes the seat but a Brooks is always period correct.