I'd take the money as well.

There seem to be an increasing number of ex Uk longhaul I class fares available. (Year round almost!) India routes for about £1.3k is an example.

So it shouldn't be too hard to get good value out of the voucher. Remember spending the voucher would probably earn a Gold 15-20k miles on a longhaul round-trip! So your net benefit from taking the miles option is only about 80k anyways...