Gee - there is some naivety on this thread. IG and CMC (and some others) generally make their best money off their counterparties. BTW - you are not their client - you are their counterparty, and their job is to take your money (until it is all gone???). Everything is carefully constructed to achieve this, and to provide opportunities for you to lose on your bets against them (you are not trading in "the markets", but in their market, which is a very different thing). There is no point backtesting etc.
You need to think very carefully about how you deal with these market making "brokers", and need to be very experienced to avoid the pitfalls. Once you have (say) ten years experience trading other platforms, you might be OK, esp if you have very long term trades, and are not overleveraged for your personal financial situation, and are not worried about losing a few ticks on closing your position due to the spread and the inability to take advantage of volatility. Alternatively if you take very short term trades when the market is very liquid as well. Also if you are a total cynic, and feel you can read your counterparties minds, you may thrive in their environment on the lower fees etc.
If you plan to trade with stop losses, you need to be super experienced. In some cases paying the GSL fees can be a great risk reduction tool, but at the same time any stops mean you have a target on your back, and if possible they will trigger it. You also will need to argue the toss on multiple occasions - helps to be somebody who loves to argue and haggle, when the "unexplainable" occurs. Also any index you trade needs to be very liquid overnight and in quiet times, if you want to be able to use a stop loss. Finally you might need to have your account well topped up if your position has periods of very low liquidity so that you will not find your account out of margin at the most adverse moment possible for your trades, as that can be a nuisance to your wealth.
Read the documentation they provide. Anywhere they use a seemingly conditional word, change it to an unconditional. i.e, might and may means will or shall, could and can means will or shall too.
