The original Williams models were numbered to confirm their status as examples of the 2500 models that were produced to conform to Group N rally homologation rules. The Williams was initially intended for a limited production run of only the obligatory 2500 cars to satisfy these rules in 1993. Prior to 1993 the homologation car was the 1764cc Clio 16V and the rally car was also a 1.8, so they had to homologate their new 2.0 rally car as a new car. In fact, Renault ended up entering Group N and A rallies in 1994 and 1995, which is why the Willy 2 and 3 came about. Just more homologation cars. It was never a standard production car. It also had nothing mechanically to do with Williams!!
UK right-hand drive models were numbered 1-400. However, it seems they are actually numbers 2-400, as number 001 (as confirmed by King Stromba) is actually an unregistered LHD car owned by Williams F1 Engineering.
The UK got the first 400 as it was considered the most important hot hatch market at the time, like Renault also did with the latter-day 172 Cup/Ragnotti. Logically, the rest of Europe would have numbers 401-2500, but in fact there are some "gaps" in the numbers as the plaque numbers actually go up to about 4000 for the LHD cars.
The Williams 2 and 3 were also produced to satisfy rally homologation rules, but were not numbered like the original series was. As we all know, the differences apart from the numbering are only minor cosmetics between the three Williams.