Yes so the 241 complicates this a little.

The process of booking the return as a one way when the seats are released is well known. After booking you can ring up and effectively get the 241 applied across to the return as well and you will get half the avios back. This method does not usually involve any recalculation of the taxes/fees/charges tho as a return, so you are left paying the one way taxes/fees/charge - something which is not recommended for a UK-US rtn, but can work out well if booking UK-HK or UK-BR. You are left with two one way bookings using this method.

The other issue here is as you found the website does not correctly price the taxes/fees/charges for an exBR booking and you would normally have to call - but of course had you called this would have been added as a return to the existing outbound, and you would not be eligible for the low exBR charges.

The problem here as noted by cws is finding someone who understands all the above and also how to escalate to get the refund of the overpaid charges.