In series is the method. All same voltage Brown to Brown and Blue to bits!!
So with 230 volt not a problem however using 20 x 12v units from a 230v supply is not possible and any extra low voltage units will need transformers/inverters.
The regulations (Part P) includes extra low voltage it does seem a bit OTT to include 12 volt under Part P but that's what it says. Personally I would think if the transformers/inverters is double insulated and indoors then there is no real danger using 12 volt outdoors and can't see why included but it is.
However if you worry about the law then cheapest way is to use a electrician who is member of scheme. There have it seems been changes to the LABC charging system and it may have dropped but it was £100 plus vat.
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