Ian, cfd's and HFT's (high frequency trading) are different tading concepts.
Cfd's are created by individual brokers and traded off exchange, ie over the counter (OTC). The broker is the counter party to every trade and provides the liquidity, matching buyers and sellers. Generally though cfd traders would hold a position for relatively short time periods, intra-day or perhaps a few days, HFT's are trading directly through exchanges in sub-second time periods.
All trading involves employing a statistically profitable strategy, consistently - bit of tongue twister there! HFT's are programmed with algorithms generally based on historic price patterns like consolidations, high's and lows, volume, time of day etc. Human traders trading cfd's OTC futures, options, or shares on an exchange, such as the LSE, or CME do much the same, only they interpret the information visually, and manually put in their orders. HFT's now control the very short time frame arena, because of their technological advantages.
Physically, because of the time taken to click a mouse or press or a hot key manual trades cannot be executed at sub second speeds, so really by definition manual trading, of whatever type, cannot be done sub-second.
That's my take on the difference anyway. I don't know how or if a proprietary HFT company has any special advantages by way of tax etc. I wouldn't think so but someone else might know more.