The stock wheel studs will get you failed because there won't be enough thread holding the nut, and therefore the wheel on, hence the need for longer threads, but as you say -ve offset rims and spacers should bring it back to spec.
RTA VSI 09 states that you're allowed to fit larger wheels as long as the wheel track (distance between the centre line of each wheel) is not increased by more than 25mm, but does not mention anything about using spacers.
An important requirement for all replacement wheels is that the wheel track must not be increased by more than
25mm beyond the maximum specified by the vehicle manufacturer for that vehicle.
Where non original axle or suspension components are fitted, the offset of the wheel in relation to the axle or stub
axle assembly used shall not be increased by more than 12.5mm each side of the vehicle based on the specifications
of the axle components used. If an axle assembly is shortened then the track width limit is taken as the axle
manufacturers original track dimension, less the amount the assembly has been narrowed, plus 25mm.
The theory behind the hub centric spacers is they have a recess that sits on the standard hub spiggot then the wheel sits on a spiggot machined into the spacer, then using longer studs from the original hub should mean that all loads should (more or less) be standard.
I may have to speak to an engineer, as the spacers would be returning the rims BACK to the standard offset instead of extending it beyond standard, and load would be beared by the spacers spiggot and in turn by the hubs spiggot, not by the studs as with cheap ebay still spacers, the theory sounds nice anyway.
Regarding your desert rats, they may still be legit but your tyres are way off, anything with more than 15mm larger diameter and/or 26mm wider width than standard is supposed to be passed by an engineer.
Jordan.
Update: Upon closer inspection I discovered this on page 4:
The fitment of wheel spacers (or adaptors for dual wheel conversions) between the wheel mounting face
and the road wheel is not permitted unless fitted as original equipment by the vehicle manufacturer.