Checkout currency

I am running a store with the following setup.

My main currency (default) is GBP and i have 8 other currencys eg usd eur, kwd, sar etc. My payment gateway does not accept 4 of those currencies (kwd,omr,qar,bhd) so i have set these to no in your checkout currency. I am also using your Fixed pricing where i can Set up product price in each currency manually, this price will overwrite the calculated price. I used fixed pricing as i bury some of my shipping costs in the price for certain currencys. If i have a product which is 130kwd this may convert into GBP via an exchange rate to £52 but what it is doing is taking the fixed price of what i have entered for gbp £42. Is their anyway to have certain currencies use the calculated price that are converted at checkout. I dont mind a hack eg creating new currencies and hiding them etc. Any way around this or any ideas that you can think of would really help as im stuck. I have created a video to explain this better https://www.loom.com/share/e9206aa6ab6b4b4f9ae8b482a0f883ff