I agree with Steve, in London I know of no other establishment like HoW.
But as PuntingKing points out there are lots of agencies (mainstream and BDSM / Mistress specific) and listing sites that provide links to SPs that are either kinky escorts or ProDommes. HoW is more the kinky escort SP as personal services are on offer (extras?), whereas with a ProDomme you will 99.9999% of the time only ever get BDSM services.
ProDommes will have either their own dungeon set up (like HoW) or use a hired space. Kinky Escorts will most likely has array of toys / implements in a domestic setting or happily go to a hired dungeon space.