I have never used my miles for a straight redemption flight, but I wonder if the following is possible ?

I am pretty sure that this can't be done online, but if I phone BA to book my redemption ticket will I be able to do this...

A USA return club redemption is 100,000 miles, a return first is 150,000. Can I fly out club and return first for 125,000 ? Is that something the system will allow ?

It's just that there are no club redemption seats on the return date I want, but there are first redemption seats.

Thanks,
Kevin.