Didds, tbh I'd just go and get the van and be sensible, i.e. wipe everything down with virus-killer spray (dettol do a good one) before touching anything in the car that someone else might have touched: door handles, steering wheel, gear leaver, etc etc. And give it a good air-out in case someone had coughed or sneezed in there. The virus can survive about 24 hrs in air, and up to 3 days on hard surfaces, so it's a possible source of infection if you aren't careful.
(Of course you are in a vulnerable or shielded group, I would seriously leave it and just stay home.)
The chance of a face-to-face encounter while making a point-to-point pickup is seriously low. You can pick up fuel using contactless if you stick to less than £30, just take some plastic gloves with so you don't have to touch the pump with bare hands (obviously discard gloves before returning to vehicle).
Keep to speed limits and drive carefully. In the unlikely event of a breakdown, stay more than 2m away from the mechanic. People still go to work and this is no worse than what they do every day, after all.
Before the lockdown I bought a breadmaker on ebay which was collection only, it's in Honiton and I'm in N Devon, about an hour's drive away. I'm going to get it on the weekend on the basis described above, and they will leave it on the doorstep for me so there's no need for any face-to-face contact. I plan to put it in a bag, disinfect my hands, and then drive home - and then leave it at least 3 days before using it, to allow time for any virus in there to die off.
The entire point of the government restrictions is to stop/slow the spread of the virus from household to household, and sticking to these principles will do just that, even if you do have to go and pick something up.
But obviously now that the rules are in place I won't order anything else that's collection-only!!
The justification for the bread-maker is in case there is a shortage of bread, or in case I have to self-isolate (again!) and can't get a grocery delivery (hard to come by these days) or friend to help.
Your description of Salisbury Plain near Porton Down was priceless, thanks for the laugh :-D I used to work up that way and you have put it in a nutshell.
Kurt, for goodness sake don't try any of that sneaking around at night nonsense round that way or you might get shot by some trigger-happy squaddie thinking you are some sort of terrorist, or they may just be doing tank practice and not notice you with your lights off.