FAIR VALUE MEASUREMENTS - Narrative (Details) - GBP (£) £ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward currency contracts	£ 37.4	£ 43.3	£ 45.0
Measurement Input Expected Term | Foreign Exchange Contract | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, term of contract	6 months
Measurement Input Expected Term | Foreign Exchange Contract | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative, term of contract	1 year