Void days add validity to a new ticket or give you a partial refund on the relevant ticket, reliability and punctuality discounts reduce the cost of the new ticket, provided it is for the same route and valid for a period no longer than the old ticket.
For renewals between 19th December 2010 and 15th January 2011, First Scotrail have Void Days on most routes for this period (16 Nov - 13 Dec IIRC), whilst First Transpennine Express, Virgin Trains and South West Trains have Void Days on all routes for the current period and further days from before that, but they vary by route and operator. You can only receive Voids Days for days when you held a valid season ticket.
I don't think the TOCs have to give them unless you claim them. All TOCs that do not operate delay repay schemes, must show Void days and discounts at relevant stations in Punctuallity and Reliability posters, they are required to be shown as part of their franchise commitment.