The tier points you lose will either be worth nothing (you would get silver next year anyway) or A LOT (mmmm - lounges) My inclination would be to forget about it and just lose the 40 points.

If you are in a position next year where you need exactly 40 points then you can book a ~£200 fare and get them.

So the choice is spend £138 now - perhaps for nothing (you may get silver through normal flying activity anyway next year) or maybe spend £200 (or perhaps less) next year IF you are unlucky with the number of tier points you get.

Doing a probability distribution of tier points from, e.g. 400 - 800 would say you'd be off your rocker to spend the cash now as the chances are very low that you will need those 40 points.