The best way to test smoke detectors is with a solo test kit.
No Climb Solo 823 Smoke & Cordless Heat Detector Test Set 9.3 m [SOLO823-101] - £1,280.58 : Security Warehouse. Security Supplies. Cheap Prices for Texecom, Pyronix, Visonic, Scantronic, Honeywell, Online Security Products, Trade Prices, Cheap Suppliers of Texecom, Honeywell, Pyronix, Visonic, Scantronic, Cooper, Dedicated Micros, GJD, Voltek, Risco, Rokonet, Dualcom.
You can just use
Smoke Sabre Smoke Detector Tester SS-01 - Security Warehouse
but be careful of the amount used as all testing sprays will clog the detection chamber if overused