There is a possible solution for you.
You'll need a second giffgaff sim:
put your main sim (the one with the phone number that everybody knows) in your best phone
on that main phone, set up divert on unreachable and divert on no reply, to the number of your second sim
put the second sim in the garden phone
The result will be that if incoming calls are unanswered on your best phone, they will go through to your garden phone. If you switch your main phone off, the diversion will take place immediately, but even if you forget the calls will divert after a short delay.
Normally, you pay for diverted calls as if you dialled the new destination yourself. But if you qualify for free giffgaff to giffgaff calls -- which you will if you have a recurring goodybag on the main phone, or if you remember to add some credit every 3 months, those calls will be free.
On the garden phone, you won't need a goodybag and a £10 PAYG credit could last many months or even years if you don't make many outgoing calls from it. But you do need to be sure to make at least one outgoing call or text every 6 months to avoid the sim deactivating.