I assume that your parents were paying a market rent to the trustees for living in the house, otherwise the gift to the trust would not have worked for Inheritance Tax purposes. Without knowing all the facts, I would say that there is no way out of the Capital Gains Tax liability on the rise in value of the house since 1999. Selling the house at a nominal value will not work as HMRC can simply substitute the market value for the sale proceeds. Recent changes in the tax rules mean that many of the schemes set up to avoid IHT on the family home no longer work - this may or may not be the case with you, but you need to get proper professional advice.
If your solicitor did not advise you of the Capital Gains implications in 1999 then he was professionally negligent. However, you need to check this very carefully before you take any action against him.