fx risk

I have held Tullow since the takeover (currently through ETrade and Commsec). I'm contemplating sale of some and want to separate market risk from FX risk. I've spoken to both my brokers about keeping sales proceeds in Sterling so I can deposit them in a local account and change to $A at a later date. Neither brokers will do it and both convert proceeds of sale immediately into $A at the prevailing exchange rate.

Would appreciate some advice from anyone who may have a solution to this issue.

Cheers. Solly.