TunsterUK
I had the same question , but I believe I have finally worked out the answer myself and can answer you from looking at my own trade history across 2 accounts. You will be charged for every trade in your first month. Rules are listed, but not well explained on https://www.ig.com/uk/investments/share-dealing/costs-fees#tradingshares - and see screenshot. The concession is triggered for the next month once you have done 3 trades in any account and any currency. For US trades the concession reduces the rate from £10 to £0 , for UK trades concession reduces the rate from £8 concession to £3. There is no concession in other international markets
I opened an ISA in April -
In ISA April I made 3 UK trades and 1 US trade and paid £8 for each of the UK and £10 for the US - total $£34
In ISA May I made 3 US trades - all free
In ISA June I made 2 US trades - both free
In ISA July I made 2 UK trades - (£3 each) , and 3 US trades (free)
I did not understand why my July trades were free (US) and reduced (UK) when I only made 2 transactions in June - but I looked at my trade history in my other account Share Dealing Account (SDA)
In SDA April I made no trades
In SDA May I made 3 US trades in May, all free.
In SDA June I made one EUR trade (€10)
In SDA July I made 3 US trades (all free), on EUR trade (€10)
So it looks like the total of 3+ to qualify for free (US) or £3 (UK), counts across accounts - it is a concession to the account holder, not to the specific account only - and it applies to calendar months . Any transaction counts to the 3 + so since I made 1 EUR trade in June and 2 US trades - all my July US trades will now be free and all my UK trades will be £3.
Your best bet for now may be to make 3 trades now and then wait will next month especially if you are planning to make any relatively small trades where the commission would represent a significant percentage. also you might try a strategy to keep the concession, once you get to free trading in August and you are worried that you have only made 2 trades you could keep the concession by making a few small trades just to qualify for September, that may work - e.g. sell a single holding on one stock you own, and then buy it again a week later (hopefully on a dip) .
Hope that helps and good luck investing