1 Book your van using Tesco Clubcard vouchers if you can – I’ve not paid cash for a booking in years. If you’re paying using real money, assume £170 or so, depending on when you travel. No idea where in the UK you are, but add another £100 for fuel on the UK side. I’ve assumed minibus @ 25mpg.
2 It’s around 550 miles at the French side. Diesel’s a bit cheaper there, especially if you don’t buy on the autoroute. Call it €300 for the fuel on the French side.
3 Tolls are circa €70 each way, €140 in total.
4 Loos on the motorway are dire – take your own toilet paper. Motorway service stations are expensive for food and awful.
5 Van hire won’t be cheap. Best to get a 8/9 seater VW, but it’ll still be £500 for a week, I suspect.
Overall transport cost is therefore looking like a minimum of £1100-1200. Not awful for 4 of you, but not that cheap either.