The only way to get tier points this side of the Atlantic is to fly.
See the VS website.
The easiest / cheapest is probably a short Business class flight with BMI.
I wonder though, if it is worth making a special flight just to keep silver?
Which of the very few benefits of the status mean so much to you?
If it was a few TPs to retain gold, I would say - Go for it!, but I find it difficult to be so enthusiastic about Silver.
Nick