As a ballpark figure I would think that you'll be looking around £ 400 plus VAT if you deliver to and collect from the RFD. If the RFD has to collect and deliver from and to you then around £ 600 plus VAT.
It may be cheaper for you to consider p/ex against an already converted revolver.
The problem is not the work, as such, but that it will have to be submitted to proof. On demand, as a one off "walk in" is going to be around £ 60 plus VAT Proof House fees. Plus the cost of the RFD (plus an accompanying person if his/her s5 is so conditioned to require one). So that is likely to be £ 200 plus VAT charge for that.
So effectively that's £ 260 plus VAT gone already. Hence the estimate that the total cost will be around that £ 400 plus VAT figure. Proof on demand is usually drop off in the morning pick up after the one and a half hour lunch break that the Proof House enjoy! So for the RFD it is a day wasted.
If the revolver has to be left at the Proof House and then collected back that "transport" cost effectively doubles....two journeys. Sometimes they will say they are too busy to do a "walk in" on demand proof.
I am sorry that this is pessimistic but from initimate knowledge of proof charges and their scandalous "minimum fee" and "plus 50% 'walk in' on demand fee" and the costs of RFD s5 transport that is a rough ballpark.
With a pistol it's just a question of (permanently) modifying its magazine in a permanent manner. And that any good old fashioned garage can do while you wait for a good drink.
On a revolver the problem is that your FAC conditions may restrict you from taking it to that good old fashioned garage and standing over the mechanic's shoulder as the work is done while you wait. The task itself isn't too difficult as a DIY task for anyone with access to a lathe and brazing or welding gear.
And subsequently, again if your FAC conditions allow taking it to the Proof House by yourself. But see below re s58 (1) of the Firearms Act that states as quoted below. So that may render any discussion of need for a condition on your FAC as redundant as s58 (1) affects that.
For lunch "The Bond" just around the corner is good, has Wi-Fi and if you are lunching has on site car parking.
In theory, in law, in the Firearms Act it does say "nothing in this Act shall apply to the Proof Houses"....but I'd ask you issuing authority for their opinion on what you can do by yourself. Some argue that this extends to being able to just post via Royal Mail revolvers and pistols to the Proof House!
This is s58(1) of the Firearms Act:
s 58
(1)
Nothing in this Act shall apply to the proof houses of the Master, Wardens and Society of the Mystery of Gunmakers of the City of London and the guardians of the Birmingham proof house or the rifle range at Small Heath in Birmingham where firearms are sighted and tested, so as to interfere in any way with the operations of those two companies in proving firearms under the provisions of the Gun Barrel Proof Act 1868 or any other Acts for the time being in force, or to any person carrying firearms to or from any such proof house when being taken to such proof house for the purposes of proof or being removed therefrom after proof.