I visited this project about a month ago, it is based just outside St Austell in Cornwall. It's an enormous project to create several different climates within 'biomes' - gigantic domes to hold plants and insects. The project is not fully complete, but is already very impressive and is continuously full of visitors.
The biomes are within a disused quarry, with the surrounding area developed as well. It is worth seeing but remember that it is not fully complete - many plants have not been planted yet, information notices are minimal, and not many insects have been introduced yet (though they plan other animals too -finches, geckos etc).
However, it has to be one of the Millennium Commission's least publicised yet most successful project yet!