Sounds like you have most of it nailed down!
I don't think the bumpers will need modifying, but the lighting will. The easiest route would be to convert to sealed beam headlights (like work vans have) then install the Hella H4 lights. You will also need to swap to the '92-'94 tail-lights and split the turn/brake circuits at the steering column switch, and also add some mid-ship turn signals.
As for brokerage fees, shop around, but most importantly select your customs broker BEFORE the van ships. If it ends up going into the lottery at the port, you'll get charge MUCH more. Keep in mind that there is a base fee, and a processing fee for each line item on your commercial invoice, so it may not make sense to pack a bunch of low-value items you could easily buy over there.
My best experience with a European firm has been with Panalpina.
My overall preferred shipper/broker though is Steven's Global. I don't know if they handle UK custom's directly however, but they have good partners where they don't have a broker.