H Huntsman's RTW was made by Cheshire Clothing, which also made for Chester Barrie, the top lines of Spencer Hart, Ozwald Boateng and Gieves and Hawkes (not diffusion line "Gieves") and probably a few others. Cheshire Clothing has now gone into liquidation, so there are no quality ready-made suitmakers left in England. Cheshire Clothing was the suit manufacturer created from Chester Barrie after Chester Barrie went bankrupt. Chester Barrie is now a brand and a name on a shop and belongs to someone else. Huntsman may now be using Cantarelli. Richard James, most normal Gieves & Hawkes and many others use a few other factories in England but their quality is nothing special.
Anderson & Sheppard does not offer RTW, nor do Poole (outside of Japan), Sedwells, Dege and most of the smaller houses.
Oxxford will have the most handwork. Its silhouette is markedly different from a British silhouette, bar its Radcliffe model and perhaps some of its Crest line. It's a difficult comparison. It is probably better made than anything you will find in England.
Kilgour's RTW suits are now made in China. They are supposed to be excellent [insert "however"].