I use 950300 which I believe is the code for construction toys - plastic. Like you I struggled to find it but there is a post here from a few years ago that mentioned it. Apparently there is another three digits you can add to be more specific, but I've found this is sufficient (by the way you'll probably need it for all international orders, not just France)
Also, if you look at the Royal Mail 'Get A Price' site - https://www.royalmail.com/price-finder - find the country and service you want, on the address screen you can type in the contents of your package (Lego) and it will display the customs code. The code seems to change slightly depending on where you're sending to.
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Rich