The largest claim that can be made is to rebuild your home to a standard identical to yours, this includes labour and materials.
Typically, the cost to do this is usually less than market value; you might find this value in your Structural/Building Survey or Home Buyers Report under 'Rebuild Value'. Remember to include garages and outbuildings and take into account any special features or materials.
If you are unsure as to whether £1million of buildings insurance is sufficient, the Association of British Insurers (ABI) offer a rebuild cost calculator to help you, or you can get an accurate calculation through a surveyor. You can find a local surveyor by contacting the Royal Institute of Chartered Surveyors (RICS).