More and more of these cases are coming about - you have two problems here.
Firstly, the fact that your parents did a Dec of Trust declaring you as the beneficial owner and them as trustees could constitute a disposal. Any disposal within 3 (I think, could be 5) years can result in the LA trying to clawback the discount.
Assuming they don't seize on that point (most of them are not awake to it - yet), whilst your parents have declared a trust, there is room for argument that they have disposed of their assets. If a substantial reason for this was to avoid care home fees, your parents can be deemed to have the capital. (You may be able to argue that the main reason was to enable you to introduce funds to purchase their house, which would probably work, but consult the CRAG document - soon to be updated).
However, I would argue that the extent of their capital is proportionate to the discount they put in - eg if the discount was �20k and the house cost �100k, they are entitled to 20%.
However, generally speaking (if the above points are not taken), your parents as trustees should be safe from the LA since they are not the true owners of the property.