I just worry as mine's a pickup and I get stopped alot. Also I worry that my wheels might fail and I'II have a catastrophic accident at high speed....whilst struggling to do 28mph up the local hill...

I worry a lot. Unnecessarily
I think twin rear wheels look quite tasty myself, kind of slot magish, there were two versions two, I think the Later or German ones look even more slotmaggish. If it were me I'd just fit a twin wheeler axle and a skirt for a van, for the front use five stud wheels on the front, to get the right offset with a dished front wheel and then fit some slotmags somewhere that vaguely match the twins at the rear. I think you'd be legal with different hubs front and rear, tyres only have to match on the same axle.
What are Transit tyres anyway? 5.5J so a full 11 inches of road holding on each side at the rear. Actually the reverse is true, I had a MK3 twin wheeler group4 van for a short while, scarily tail happy, as bad as a chassis cab, just a chassis cab mind -without any body on it!

My Mk5 was biodegradable and disintegrated with time.
Ford. Committed to the Environment
Finally entered the 21st Century and bought a MK7. Oh Dear.