Hi and Welcome,
Im in the UK too Hertfordshire, there are some great Auctions out there but theres also stiff competition, be prepared for early mornings,long drives,and standing in a saleroom all day to find the thing you really really want to bid on,goes well above your price range, ive been doing that very same thing for many years,
These are some of the better Auctions in the South which have Textile Sales, Kerry Taylors which you mentioned , then Wellers Auctioneers in Surrey,Dukes in Hampshire, and the Coltswolds Auction company in Gloucestershire, they are mainly 2-4 times a year , always a varied lot of Vintage clothing,
But i would also recommend going to some of the Vintage Clothing fairs, Mainly in London, but always worth the drive to try and find that one or five special items.