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Simple Summary: Atypical scrapie (AS) is a disease of sheep and goats. Currently, when AS is detected on a farm in Great Britain (GB), all animals that exit the flock through natural death or slaughter are tested for AS for a period of 2 years following detection of the initial case, as used to be performed within the European Union (EU), in order to provide support as to whether AS can transmit directly between sheep. The aim of this study was to determine how many AS animals would be missed if this testing was also stopped in positive flocks in GB and hence determine the value of continuing with this surveillance programme. Our model estimated approximately one undetected AS case every three years if additional testing was stopped in detected flocks. In comparison, around 10 AS cases are detected per year through other active surveillance streams. Our model estimates suggest that stopping the intensive monitoring of AS would have relatively little impact on AS surveillance and that continuing it would provide limited evidence as to whether AS is contagious. Atypical scrapie (AS) is a transmissible spongiform encephalopathy (TSE) that affects sheep and goats. Low within-flock incidence suggests that AS is not transmissible between animals, and testing of all animals that exit positive flocks for two years following detection (i.e., intensified monitoring) used to be carried out in the EU to provide data to test this. This intensified monitoring stopped in 2021 but continues in Great Britain (GB). The aim of this study was to predict the number of AS cases missed if this monitoring were also stopped in GB, using a combination of statistical and transmission modelling. The number of AS cases estimated to be missed if the intensified monitoring was stopped was low relative to the number of AS cases detected in other active surveillance streams (e.g., fallen stock and abattoir surveys), at approximately 1 case every 3 years (0.34 per year, 95% CI: 0.18–0.54) compared to 10 per year (95% CI: 4–17) in the active surveillance stream. This suggests that stopping the intensive monitoring of AS would have relatively little impact on AS surveillance and on the power of the available AS data to infer whether AS is contagious. [ABSTRACT FROM AUTHOR]