Back 40+ years ago, Bike Warehouse (now Nashbar/Performance) offered this bracket that kept the front of the rack from sliding down the seat stays:
( Edit) now I see that you're looking for a specific piece to fit a particular rack to an English bike....
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'75 Fuji S-10S bought new, 52k+ miles and still going!
'84 Univega Gran Tourismo
'84 Univega Viva Sport
'86 Miyata 710
'90 Schwinn Woodlands
Unknown brand MTB of questionable lineage aka 'Mutt Trail Bike'
Plus or minus a few others from time-to-time