I had my Vauxhall Combo MOT'd this week - it failed on several things including the rear coil springs.
I've had the coil springs upgraded to heavy duty & what a difference it makes to the handling of the van with a full 350L tank... Wish I'd upgraded when I first purchased the van.
I asked a garage about upgrading the rear springs on my connect when I first got the van and I'm sure the garage bloke said that doing that would affect the breaking as the depending on load there's something that adjusts the amount of brake force to the front and back brakes to take the loading into account on the front/rear axles.
If the springs are upgraded then that won't work properly and it'll fail an mot or something like that. Maybe it's the same for your van.
I may have remembered it wrong but at the time I remember there was a good reason given.
Older trucks like my Ranger 2 have Load Sensing Brake Bias Valves fitted. They can tell by the weight of the load how much braking effort to apply to the rear wheels.
The main issue here is that if the load is 0 kg and the braking effort applied is that which is required to stop the vehicle with a 1,250 kg load then the rear wheels will instantly lock up and you'll merrily slide sideways down the road at a high rate of speed.
If you fit uprated springs then due to the lack of downward movement on the rear springs / rear ride height the load sensor is unable to determine how much weight is present and therefore unable to determine how much braking effort to apply.
I unwittingly uprated the rear springs on my Ranger when they needed replacing. I ordered a standard set from a Road Spring Company and then found they gave a much harder ride than before. Braking is unaffected however as there seems to be a certain amount of leeway in the sensing.