If I spend £100 to make £101 but I earn that £100 from the farm then it's all counts only if that £100 came from outside farming would it not. If a shop purchased £100 in stock and sold it for £101 then your right only £1 income and net benefit. But we generate the money we spend by growing crops, not buying crops and selling them on. All the money we spend comes from the land, that includes the 16billion in running costs. Without farmers that 16 billion goes as well. (And so does all the jobs it supports)which based on £20k income is around 800,000 jobs in the support chain.I repeat, money spent is not value added. If you spend £100 to get £101 in sales, you've added £1 of value not £100.
And if you cared to peruse the figures I linked to you will see that the entire gross output of UK farming (ie value of everything sold, crops and livestock) is £25bn. Not £100bn as you suggested. Out of that £25bn in receipts, farmer pay out about £16bn in costs, leaving the £9bn in net value added. Thats it, in black and white. If you don't believe it go and take it up with the Office of National Statistics that you think they are undervaluing the contribution of the farming sector to the UK economy by a factor of 10.