Following a (much) earlier comment on the merging i.e. "speak to a human and they will merge your mileage" I tried this for our flight last month ( now back in lovelysnow) only to be told it cannot be done.

Due to a 12 hour+ delay a couple of years ago we were given 25000 miles each; which we thought was OK. SUbsequently, and as posted you can only use 2000 at a time which makes the ;offer' of 25K in compensation for the inconvenience" virtually useless, even going 2 times a year.
SO we booked Prem Econ this time[clap] with the human for the first time and used 7500 miles each.
Next day we looked on the Virgins site and it was £200 cheaper to book online! Moral is/seems to be- dont talk to a human UNLESS you have a pre arranged list of questions worked out before hand!