Pantherella socks (Aylestone), Albert Thurston braces (Frog Island), and Mars Knitwear (Wigston) are all based and produce locally. Origin knitwear I think use Mars to make their stuff.
Paul James knitwear also produce some stuff locally but not all.
Wolsey knitwear (now owned by Matalan iirc) were also founded/based locally but not anymore.