The Standard Variable Rate is the rate which many mortgages revert to after the introductory offer fixed rate or discount period is over. They are the simplest and most traditional mortgage product with no upper or lower limit on the rate charged and the bank can raise or lower the rate at their discretion (though usually this is done broadly in line with the base rate).
Mortgage relying on Bank of England's base rate.
The mortgage lender's interest rate at any give time on mortgages that are not under fixed, discounted or capped rate conditions. This is the rate that the mortgage will have once the special conditions expire.