...
Buy Jeep for 20k - trade in car for 10K - balance paid 10K
Get 4,200 back in VAT so real cost of upgrade 5,800
Tax, insurance, go down as business expenses.
Diesel for business go down as expense as well...
I'm not sure I understand your sums fully, but subject to correction, how about this:
DH sells car for 10k and gives business 10k loan or registers debenture
Business buys van for 20k
Get 3,471 back in VAT, so real cost of upgrade 16,529
Tax, insurance, maintenance go down as business expenses
Insurance will have zero NCB unless you have had commercial insurance previously
Diesel for business goes down as business expense as well...
VAT on diesel and maintenance reclaimed by business
Van as depreciating capital asset gets written down year on year before business tax calculation
Loan, subject to cash-flows, gets repaid before business tax calculation
BIK paid by directors for private use of van, if such situation arises