LiveinjapanI think 'troop cuts' also possible.
Then, your sentence will have to be revised as "Britain announced last year it planned
to cut troop numbers a troop cut to two thousand and five hundred but the government delayed the move."
In "troop cuts", 'troop' is used as an attributive (adjective) distinguishing 'cuts' like "power cuts", "pay cuts", "staff cuts", etc.
I list below two dictionary entries for clarification:-
Webster's Revised Unabridged Dictionary -
To cut down--To lessen; to retrench; to curtail; as, to cut down expenses.
Dictionary.com Unabridged
cut down,
| | Also, cut down on. to lessen; decrease: to cut down on between-meal snacks. |