I'm afraid you are wrong. The situation is the contract is NOT voidable. There could never ever be a contract.
For a contract there has to be three things
1. An offer - here I offer you £1 for your car
2. Acceptance - you accept the £1
3. Valuable Consideration - I pay over the £1.
The law is absolutely clear this makes the contract.
You cannot void a contract because you decide later you have made a bad deal.
To quote from one of the recognised legal textbooks Contract: The Law of Contract (Common Law Series)
Another legal textbook Goode and McKendrick On Commercial Law provides:
In the example I provided there is no misrepresentation, no undue influence and no duress and therefore the contract is not voidable
I would be interested to see your caselaw that you claim exists to support your claim that a contract is voidable because one party later decides they do not like the agreed contract.
You may want to look up the case Newbigging v Adam which provides Contracts are not voidable for mistake
"You cannot void a contract because later you decide you have made a bad deal"
It depends on the circumstances. What we are discussing is selling something obviously valuable at far less than the market price. The issue is whether the beneficiary under the contract (the purchaser) may reasonably understand that the agreed price is far below fair market value. If they do, then they are on difficult ground to enforce the contract
Consider the following circumstances:
1) The individual who sold the car has dementia. There was no misrep, undue influence or duress. However, in selling it for £1 the contract is voidable
2) The individual is not native to the country and didn't understand the currency. Again no misrep, undue influence or duress. Again, in entering into a contract demonstrably far below market price and with a reasonable explanation as to why they erroneously did so, contract is voidable
3) The contract was written in a way that an ordinary person may not reasonably be expected to understand what was being entered into. Such as a normal sale price but, hidden away in the small print, some huge commission. Again, voidable
4) The individual was a minor who had inherited the car from their aged aunt