The taxes/fees/charges will be those for a LHR-ORD/PHX-LHR fare, and this will not be the same as simply the addition of TFC for two one ways. It should be cheaper as a return as you have found. In fact for a UK-US rtn trip I would strongly suggest you do not book it as two one ways as exUS charges are very high.

You should be able to find out the TFC by pricing up a cash return for your flights on ba.com using the multi city tool, and then looking at the price breakdown. The TFC should be the same when you book it as a reward. You will have to call to actually go ahead and make the open jaw reward booking as this can't be done online.