hart I don't know about, but Power are one of the biggest for dance music - hard list to get on, you must be working regularly and to a lot of people, and you have to send them your chart for about 6 months before you get anywhere - oh, and you have to pay them! I think it is £25 a month - you get about 100 mailouts a month though
euro solution is part of music house, which combines Hyperactive, Renegade, Euro Solution and a few others. quite big.
intermedia are radio promotions I think.
Rush is ok, quite small, you may get on this.
upshot is a bit leftfield if you like that sort