I’d echo what Georgi said to an extent, although rather than being based on the size of your portfolio I think the applicable FX rate in the current month should be related to the volume of trades made in the prior month.
<=5 trades in prior month = 0.45%
6 - 10 trades = 0.35%
11 - 20 trades = 0.25%
20+ = 0.2%
I guess there would also need to be a minimum qualifying value for the trade to prevent people gaming the system & making 20 trades for $1
The other option is as per Vlad’s post, paying something like £5 per month for 0.1% fx rates as part of a custom package…