Letchworth, in Hertfordshire, pre-dated WGC by a few years.
However, you could cite Saltaire (Titus Salt, 1856), near Bradford as the earliest example of a town laid out on rational lines, with wide (for the times) streets, green space to hand and facilities such as public baths (for personal ablutions, rather than for swimming) libraries and schools.
Bourneville (Cadbury) and Port Sunlight (Leverhulme) are later C19 examples of the same idea.
To look back to the beginning of the C19, to New Lanark Mills (Owen - c.1810 - cain't remember the exact date) is probably stretching the connection too far - although the idea of employee welfare is common to all of these schemes.
Letchworth and Welwyn Garden certainly took the whole idea up to a new level, though. The major dofference was that this was municipally led, for all citizens no matter where they worked, rather than a company led thing, which owed a fair amount to the perceived need of employers to control their work-force.
