There are the usual old time suspects like Bob Jackson, Mercian, Ellis Briggs, Woodrup etc along with more modern guys like Shand cycles here in Scotland and countless others.
Old style lugged and off the peg at a reasonable price you cannot go past Bob Jackson. better value than most and can be had for the same price as any massed produced 4130 frameset sold in most shops.
I personally find that some of the newer builders see it as some sort of boutique type endeavour and so charge well over the odds for a 531 style lugged steel frame, sometimes nearly three times the price of a £600 Jackson frame.
Jamie