Unfortunately, these allocations are
requests only and do not guarantee you those seats.
Although the general line is that the requested seats are yours unless there are 'operational reasons' to change them (i.e. change of aircraft layout) our personal experience is that we've had the 'allocated' seats less than half of the times we've flown with Virgin. There are others here who have had a much better success rate than that so, hopefully, you'll be one of the lucky ones

You can make seat requests online if you've booked with another agency, but you need to make sure you have the
Virgin booking reference rather than the agent's own reference number.