IAS is the way to go I think and it's worth checking their betting options for what personaly suits you. Betezy is OK if you don't win too often [natorious for closing winning accounts].

Top ******. can be a bit of a furphy as IAS [and others] have runners who knock down prices before official Top fluc, by betting down any horse that has attracted money [if the horse wins they win even more!].

Betfair is a dead loss unless you like sitting at a PC screen [Saterdays are too short for that].