How can I replace a non-standard (20cm) bathroom faucet?
I see that it is hard to recommend a robust solution for you because of several missing pieces of data. Here is a summary of questions that really need to be answered -


Are your current 200mm centered faucets each with their own water spout?
Does your sink have two holes or three?
When you say "mixer valve" are you referring to a single handle
faucet or one with two handles but with a common water spout?
Is the space where the faucets are located and in between them a
nice flat surface on the back of the sink?


If you have a two hole setup then you may be faced with either replacing the sink or boring a third center hole in the sink. A faucet like that pictured below is a center hole mounted unit that requires a 1 3/8" to 1 1/2" hole. It comes with an optional deck plate that can be used to cover a previous 2 or 3 hole installation. (You would have to check though if the deck plate with this one would cover over the your existing 200mm spaced holes). 



On-Line retailer link:

http://www.faucet.com/danze-d225525rb-ac0fw1-oil-rubbed-bronze-single-handle-centerset-bathroom-faucet-from-the-aerial-collection/f1072590