Someone posted recently three 2.5 FTE in parallel feeding a shower, that nobody thought much of. But that's 7.5mm so what was wrong with it? :coat
Parallel conductors are permitted but careful consideration has to be made of the fusing of cables..
especially where 3 or more are employed..
the post you refer to with the 3x 2.5mm was that off a 40A MCB? or a 32A....
e.g. say 1 x 2.5mm has 20A+ capacity so 3x 2.5mm should be good for 60A total load across all three 2.5mm's
Forget the normal running load currents and consider a Live-Earth fault in a Non-RCD protected single leg of that parallel circuit?
Say if it were off a 50A MCB how much can one of these 1.5mm CPC's carry? would the MCB trip first or melt the CPC?
Think of the potential overload issues with a broken ring circuit with some conductors not continuous..
Appendix 10 gives more detail.. where it gets complex individual fusing of legs has to be considered...
Back to the OP's question...
Excluding ring circuits.. in domestic wiring..
there is virtually NO valid reason for not using appropriate cables for all circuits...
(if shops are closed... wait until you can get correct materials!!!)
In my book this comes under.. Good workmanship & proper materials reg 134.1.1
as opposed to botched up cowboy make do work practices!