Because the supplied connection is a power supply with an integrated UK plug then the easiest solution would be to install, safely and in accordance with the wiring regulations, a light switch to supply and control a 30mA RCD protected socket outlet labelled to indicate the use only for the decking lights.
To cut off the power supply and hard wire the lights would require a 230V to 12V LED driver, but the specification of the driver (ac, dc, constant current, constant voltage) would be down to the specification of the decking lights. Whether you need the 3A fused spur or not would depend on the rating of the circuit from which you were supplying the power.