I believe that you also have to subscribe to ICE market data which costs 1 GBP a month. So in TWS, you would have to convert some USD to GBP and then subscribe to ICE market data thereafter. Unless you trade Brent IPE, I would stick with trading the EuroFX on GLOBEX instead of the USDX on ICE. 57.6% of the USDX consists of the Euro anyway...