I also redeemed miles during the 50% sale in November for travel in June and later changed online my travel dates. The taxes had dropped at that time, but I got no refund. I wrote to BA (where is BA helper?) and here is the answer :

Dear LearningToFly,

We have receive your query about the reduction of the fuel surcharge.

We have reduced the fuel surcharge because the price of oil has gone down.
However we are not offering customers a refund on ticket they have already
bought. Our policy is that once a ticket has been bought any changes to the
surcharge are not passed on to the customer. We feel this is fair ,
bearing in mind that fuel costs remain a challenge for all airlines and
while there could be a decrease, as there is now, there's also the
possibility the fuel surcharge can go up again.

I realize how disappointing this must be for you but hope I have managed to
explain the background.

Sincerely,

Jacob Weiland
NA British Airways
Customer Resolutions

Few years ago, I had to cancel a reservation and got immediate refund of my miles, but had to write and call and beg many times on a 6 months period to get my refund of the taxes paid. This is a different issue, but it also shows a certain attitude.