You cannot run 240v lights on 12v. You can, however, run 12v lights on 12v.
The aim seems to be to run safe voltages into your garden.
You can connect some of these in series to raise the voltage, but I would strongly recommend not exceeding 3 in series, for 36v supply. Any more than that and we are at a voltage that most authorities would deem needs proper insulation.
I doubt that 3 will be enough for your garden. If you do use strings of 3 in series, you will end up with a hybrid series/parallel system that may be difficult to run, wire or maintain. In which case, it's probably best to stick to a 12v all parallel system.
The bulbs you linked to are rated 3W. This means they should take about 250mA each. Conecting (say) 40 in parallel will result in a total draw of 10A at the transformer. This is about the most current you want to handle with reasonably sized wires.
Wire has two ratings for current. One that will cause it to overheat, and one that causes excess voltage drop. Although 1mm2 wire is rated at 10A for mains use, which means it won't overheat at 10A, you may find the voltage drop of any length of it to use an excessive fraction of your 12v. For instance if a long run of wire dropped 6v, this would be negligible compared to mains voltage, but would halve the voltage supplied to your 12v bulbs. In this case, you would want to use thicker wire, or split the bulbs into several smaller sets, each taking less current.
Note that if you did use strings of 3 for a 36v common supply, the same voltage drop would be much more manageable.
If you were to run 40 lights, you would want a transformer rated at least 120 watts, with a 10A 12v secondary. Anything with more current, more VA would be OK.