The basic concern is that it should not be possible to overload any given cable run, even if someone (for example) cobbles a 3kW heater onto your spur. Provided the fuse or breaker will disconnect in such a case before the cable is seriously overloaded, in principle it's OK.
As always you need to think not only about what you are installing now- even though it only draws 40W- but what some idiot might add to it later.
Exactly the same argument about future idiots can be made no matter how many FCUs he uses - he could install dozens, one for each lamp, and your concern would be just as valid, or invalid, if he installed none.
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