Surely you have only 'purchased' the rifle 'conditionally' - ie you have paid him for it but full ownership and title does not pass to you until you have taken possession of the rifle, under the understanding that this would only take place when your firearm certificate has been varied accordingly.
In effect, you've 'paid him for it', but you haven't
actually 'purchased' it until its complete - that was obviously the agreement from the start, although it may have remained an unspoken assumption, but the seller knew you were waiting for your FAC to be varied, and was happy to ship it to the RFD to be held pending completion of the deal.
Edit - in fact, legally, any firearm transfer can ONLY take place face to face, this being the function of the RFD so the purchase of the rifle is not completed until it is handed over, purchase=transfer of ownership - you can't have purchased it yet, as it would be illegal to do so, the purchase can only be legally completed with a face to face transfer:
1997 firearms act:
32Transfers of firearms etc. to be in person.
(1)
This section applies where, in Great Britain—
(a)
a firearm or ammunition to which section 1 of the 1968 Act applies is sold, let on hire, lent or given by any person, or
(b)
a shot gun is sold, let on hire or given, or lent for a period of more than 72 hours by any person,
to another person who is neither a registered firearms dealer nor a person who is entitled to purchase or acquire the firearm or ammunition without holding a firearm or shot gun certificate or a visitor’s firearm or shot gun permit.(2)
Where a transfer to which this section applies takes place—
(a)
the transferee must produce to the transferor the certificate or permit entitling him to purchase or acquire the firearm or ammunition being transferred;
(b)
the transferor must comply with any instructions contained in the certificate or permit produced by the transferee;
(c)
the transferor must hand the firearm or ammunition to the transferee, and the transferee must receive it, in person.
(3)
A failure by the transferor or transferee to comply with subsection (2) above shall be an offence.
There you are - transfer of ownership only takes place when you present your certificate at the RFD and he hands it over, and it has to be face to face by law, so you can't have 'purchased' it yet!