Giffgaff is a mobile phone service owned by Telefonica, who also own O2. The services they provide include pay as you go SIM cards as well as bundles called “goodybags” and “gigabags”. From 2014 they became the official Apple carrier.
01 numbers are described as geographical numbers for different area of the UK, and they can be bought here. The price from a mobile SIM from Giffgaff varies from 3p to 55p. Note that this is an approximate value. If you are calling from abroad it may be more. It may be possible to get free numbers as part of your call package, it is always best to check with your provider.
It is a good idea to contact Ofcom if you have problems with any aspects of your phone bill or Giffgaff’s call charges.