I have a Thorn and Hewitt, I got excellent service from both, each bike is fantastic in its own way, both require the same number of spacers for the same riding position.
I doubt there's much difference in quality between the Thorn Audax and Hewitt Chiltern (which BTW isn't exclusive to Hewitts) both neatly TIG welded in Taiwan. The Thorn is probably a bit lighter, comes in a tougher finish, is available in more sizes and comes with a long money back trial period. The Hewitt frame is £100+ dearer, for that you get to choose the colour (and it is a quality finish) get a first rate fitting service and if you're going for a bike rather than frame you get a better choice of components without the price penalty of upgrading.
You pay your money...