This really depends on what size you trade as most brokers in our index broker comparison tables tend to operate on a hybrid basis, hedging larger clients and not smaller ones.
Brokers tend to take the other sides of a trade because (apart from the obvious commercial reasons) it’s hard to cost effectively hedge every singe small index trade.
If you are a small trader it shouldn’t really matter as all the brokers in our CFDs comparison tables have low latency execution. However, if you are a larger “professional” trader you may want to consider trading futures instead of CFDs.
With Index futures you can get direct market access to the exchange order books so you can work bids and offers inside the spread.
InterTrader make a big deal about being a straight through broker, however, you won’t get the experience or support you will with a hybrid broker like ETX or IG.