I think that 70-80% of the food in the UK is imported so I don't think the impact is that negligible. While most of the money is spent on local things as you said, if after housing / healthcare / education / ... you are left with a fixed amount of money to buy groceries, a 50% price increase is huge on your QoL and purchasing power.
So if the transportation will be twice expensive, it will translate into price increase only of 10%.
Which can lead to riots. Imagine the worst off, the people making minimum wage and similar, who see their food increase in price 10% across the board. The spark for the Yellow Vests in France movement was a small increase in taxes on fuel : `tax increase had been 7.6 cents per litre on diesel and 3.9 cents on petrol in 2018, with a further increase of 6.5 cents on diesel and 2.9 cents on petrol planned for 1 January 2019`
Those protests raged for more than a year, and were stopped only by the pandemic, and caused billions of euros of various damage and have left multiple people dead and wounded.
And fuel, as vital as it might be in rural areas to move, is significantly less important than food which even people who can move around on bike or public transit need to survive.
By that logic there should have been a civil war after 2008.
The changes proposed would provide jobs to the worst off, that's a fairly reasonable trade, which you may or may not be inclined to take
The percentage sounds about right, but in money that’s £11.5 bn out of a £2038 bn GDP.
https://www.gov.uk/government/statistics/food-statistics-poc...
https://www.wolframalpha.com/input/?i=uk+gdp+in+GBP
That said, Brexit is a live demo for many of these issues.
If other things increase in price, people have less money to spend on housing, rents go down, which leads to house prices going down.
London has the worst [years of wages] for 1 square meter of housing ratio in the world. Excluding Hong Kong.
Around 1% would be my guess.
I think my average yearly spending on imported goods over the last few years was around £1000.