Personally I dont think that the mechanics at the dealers would bother about whether or not it had been lowered, in fact, I think they would admire it, and probably think that your car looked better than the standard Clio! My last car had been lowered with a Scorpion exhaust etc etc and it had a couple of warranty jobs done on it in its early days, and the Pug garage guys used to love it and never said anything about the warranty being affected by it.
I think that as long as "Renault UK" arent involved in any problems, the mechanics who would be working on it arent going to bother.