I wouldn't recommend a flat rate broker. They will be OTC for equities ( assuming you want to trade equities) and you will suffer all the problems associated with OTC trading. Stick to DMA it won't cost you more- even if the commission appears to be more so.
I have traded with IFC Markets. The company charges a commission of 0.1% for opening positions on CFDs on Stocks. In addition, they provide VIP accounts, where the conditions for the clients are very flexible and may be discussed with personal manager.