The first option is the best, if you can do it.
The RFD route is next safest but probably the most expensive (because you have to pay the RFDs’ fees and the shipping cost between them, as well as get the gun to/from the RFDs at the start and finish).
As account holders, RFDs have special concessions that allow them to send guns between each other using Parcelforce and other couriers, backed by full insurance for loss or damage.
It is legal for private individuals to post air rifles via certain services of Royal Mail and Parcel Force.
For full details of the right services and specific policies and guidelines, check out this:
Shipping Air Rifles Guide