It depends on axle weights, not overall van weight. You would have to check your axle weights on a label under the bonnet behind the air filter box, but a T28 panel van usually has 1600 front, 1550 rear I think from memory, so a 100 load rating would meet the requirements of both axles.
I would advise you to double check that though and not just take my word for it!
You will only find a 102 load rated tyre in the 255/35 20 size in the real budget brands. None of the mid range or premium offerings will do that size.
A 255/40 will give you 101 load rating and would definitely be rated high enough for your van, so perhaps look at that size instead.