Looking through the AT4 Custom Shop Gear Models PDF:
Brit Silver
Based on Marshall® Jubilee™ head
Based on the 1987 Marshall Silver Jubilee commemorating the 25th anniversary of the Marshall company, the Brit Silver features the 3-gain mode of the preamp circuit “clean”, Rhythm Clip and Lead. The Silver Jubilee was also the first Marshall to feature an output power section that could be switched from full to 1/2 power, thus providing that JCM800 tone at reasonable volumes. The Brit Silver faithfully recreates the original Silver Jubilee down to every minute detail for hyper-real authentic Marshall tone.
JCM Slash
Officially Certified Slash-Marshall® JCM SLASH 2555SL Model
The Marshal®l JCM Slash was designed to the specs of Slash’s original 1987 Marshall® Silver Jubilee 2555 amplifier. It was the first amplifier built by Marshall as a signature model.
But, oddly, the first one is NOT available in the current Custom Shop. As far as I can see, it is included in AT4, AT4 Deluxe, AT4 Max, AT5 and AT5 MAX.
And I could not find the British Collection for PC/MAC, only for iOS. I have asked Sales Support.