They are in existence but not in this country. I looked into this a while back, and was in touch with Rotrex (supercharger producers). No-one has done this job in the UK, so as such, any attempt at fitting a s/c to the Williams car would be the prototype, and set you back approximately £7k
GMC did actually tell me they would attempt the job tho! E-mail from them from June this year:
"Hi, no one has made a kit for the clio, we looked at one mk1 and it seemed a tight space to do easily, not saying it cannot be done. Our fully developed kit for the saxo is £5100, to do another car from scratch is likely to cost circa £7000 due to all the lost time making things. If you want to pursue it give me a ring on the above number."