Hi there, usually it's not a big issue to change a ticket at VN. The fees depends on your booking class as you can see here.

In your case the problem could be, that there are no more seats available in your booking class and so you have to pay the difference between your booking class and the one which is available. A look on the VN website shows, that on 2/1 from SGN to LHR only Economy flex (the most expensive economy fare) is still available.