If you want/need a 3.5 ton gross box you must be realistic about pay loads.
1.
One of my favourite sayings is "horse safety has been sacrificed on the altar of pay loads"!
(Yes I am a rather boring person) Seriously though there is a fine line between the two.
To lighten a box somethings have to be sacrificed, like load guides, beefy partitions , steel reinforcements etc etc all design features that kept horses safer when travelling.
2.
A box with all the requisite safety features would probably weigh 2.5 -2.6 unladen giving a pay load of around 1 ton or less. A box that has been sensibly lightened by the use of modern materials but retaining some of the more essential safety features may weigh 2.3 - 2.4 ton unladen thus giving a pay load just in excess of 1 ton.
3.
Pay loads are first and foremost a legality issue (3.5 ton boxes come under the white van legislation) They are not necessarily a reflection of the weight the structure can safely carry or the engine realistically pull. Renault Masters are rated at 4 ton gross in many other countries. In this country they can be up rated to 3.9 gross (the older models can go to 4.2 gross) with the simple expediency of rubber mounts fitted to the rear springs (a 20 minute job for an experienced mechanic) and a speed limiter plus weeks of struggling with red tape and bureaucracy!
4.
Once a box is up rated it will need to be plated every year at a VOSA station - no longer coming under the MOT umbrella. Also you will need to have a driving license pre 1997 or have taken the next test.
5.While not wishing to encourage anyone to break the law I do think this issue has been very much "talked up" by the horsey press and it is becoming a horsey obsession. The legislation is there (quite properly) to stop people from flouting the capabilities of the 3.5 ton vehicle and making it unsafe and a hazard to other road users. However common sense should also prevail, horseboxes should not be driven when loaded at reckless speeds which is the main rationale for the legislation. (any over loaded vehicle driven in a reckless manner or at high speeds will have its handling compromised) and the VOSA inspectorate are looking for drivers of these vehicles who are flagrantly over weight and flouting the law.
6.
There is a section on the VOSA web site concerning the penalties exacted on overweight vehicles.
My understanding of this section is that you are allowed up to 5% overweight with no action taken.
Between 5% and up to 10% you would be liable for a £60 fine and no penalty points.
Over 10% becomes more serious but over 10% on a 3.5 ton box would, by my calculations put you over 3.85 gross so you would be nearing the safety limit in any event which I know most of us would not want to exceed for the sake of our horses' safety.
On an up rated box to 3.9 ton gross the same percentage calculations would apply - but I will leave you to do the maths!