This thread has drifted away from the original question of which fleets didn't last long to references of individual vehicles that were scrapped after collision damage. On that basis, the class 66 that slipped off a dock crane whilst being unloaded would be a contender.
Are you referring to 70012? I don't recall a class 66.
Disregarding one-off prototypes like "Lion" and DP2, loco classes that lasted ten years or less:
10 years - Class 16 (1958-1968) ten members
10 years - Class 21 (1958-1968) (58 members - twenty examples were converted to Class 29, see below)
10 years - Class 28 (1958-1968) twenty members (D5705 survived longer in departmental use, and was subsequently preserved)
9 years - D3/1 (D2900, (no TOPS classification) - (1958-1967) fourteen members
9 years - Class 17 (1962-1971) 117 members (D8521 was in "Departmental" use until 1978, D8568 is preserved)
9 years - Class 41 (D600 Warship) 1958-1967, five members
9 years - Class 74 (1968-1977) ten members, converted from Class 71, introduced in 1958
8 years - Class 29 (1963 (prototype)/1965 (the rest)-1971). Twenty members, converted from Class 21
6 years - D2/12 (D2510 class, no TOPS classification) (1961-1967), (D2511 preserved)ten members
6 years - Class 48 (1965-1971) five members, subsequently converted to Class 47 specification
5 years - Class 14 (1964-1969) 56 members
Others of interest
Class 22 (Baby Warships) 1959-1972, a similar lifespan to their diesel electric cousins (Classes 21/29) but as none of them received new engines they were not reclassified.
Class 23 (Baby Deltics) escape this list because two of them lasted until 1971, when they were twelve years old. However, during their short lives they were laid up for almost two years before being rebuilt.
Class 30. Introduced between 1957 and 1962, they had all been converted to Class 31 specification by 1969. I don't know if any individual examples lasted ten years in original condition.
The North British Locomotive Compony seems to be over-represented in this list, as they were responsible for six of the classes listed (D3/1, 16, 21, 22, 29, and 41). Paxman engines appear in five classes (Classes 14, 16, 29, 74, and all but two members of class 17)
Data from "Diesel & Electric Loco register" 2018 edition, published by Platform 5