0.1% is a fantastic rate for any online trader receiving Direct Market Access. At $8 AUD per trade flat fee regardless of size you will not have DMA and hence not be guaranteed to be filled at the best possible price available.
0.1% of £5000 is £5 NOT £50. Most online brokers will have a minimum ticket charge in place of around £10 hence in this instance the small deal size you mentioned would carry a cost of £10, the minimum charge to any online customer.
There will always be a reason why a company is charging you a flat low cost commission rate to do business. Typically either spreads are widened or skewed against you. I can guarantee you the brokerage in question here that charge you $8AUD a trade do not let you see the order book and trade directly with the exchange with full DMA.
Theres a saying ''Buy cheap,buy twice,'' and always a reason behind somebody offering lower costs.
IG and MF are not the size they are by over-charging clients. You get what you pay for.