You have lost several hundred pounds I am afraid.
To comply with the OFCOM requirements to disconnect and recycle unused numbers all mobile networks will disconnect numbers and accounts where they remain unused for a specified period. Some networks do this after 90 days, giffgaff all 186 days.
giffgaff send two messages prior to disconnection, which is irrevocable. Once a number is disconnected and thrown into the recycling 'pool' no operator can retrieve it and it will be re-issued at random in 6-12 months time.
Sorry, but the is absolutely nothing that you can do to recover the number.