Are movement trays legal in Warhammer 40k Tournaments?
From what I've seen on forums and general consensus on the web, there doesn't seem to be any rulings against Movement trays in Warhammer 40K. The only issue it may cause is with certain units with blast radius weaponry. This would possibly deprecate you game if your models are locked in a tray.

However, Termagants should be okay to place in trays as from what I can recall of 40K they don't have blast radius weaponry.
Check this page and this page that I found on the matter. They all seem to suggest its legal but not advisable as it could put you at a disadvantage.