I am not an ActivTrades trader but I know exactly what these people are doing. They are using one of several so called latency arbitrage softwares. The most popular is Westernpips, the company hosts such a range of tools to steal from brokers that they even made a webclicker to mask trading via EA :
m.westernpips.com/Web_Clicker.html
This is not an HFT system or an EA at all. It is a latency arbitrage that they admit openly even on their youtube channel. What they do is compare price feeds until they find a broker has due to connectivity a slower change of quotes to the LP feed.
Then they storm a broker with connectivity issues until they rip them off.
The issue with this is that these so called profitable trades that might appear on MT traders terminal are not present on the Margin Account of the Liquidity provider often, and if they are, Liquidity providers will often reverse these trades on the broker causing a total loss for the broker.
Now, note these guys aren't posting their trading history. This is because you will see thousands of trades most of which exited for profiting with 5-10 seconds and a return of 2000-3000% within a month. I believe people that are real traders are reasonable enough to know this is impossible when trading legitimately. The best hedge fund in the world makes 50-60% returns a year, read - Renaissance Technologies.
Make no mistake, what these people are doing if theft and it is the same as trying to go in a casino and cheat the slot machine with external tools that make you win always.
The reason the ActiveTrades employee said to go to the FCA is, because the FCA are no fools and they will threat this as fraud.