Your explanation is still a little fuzzy, and I think your overall design may have some flaws. I have an idea what you need, but first I'd like to clarify a couple of things.
There is no column labeled "quantity" so maybe you means the "grams" column, because that has formulas. You say that if you input values yourself, the values stay there, so it must be that you are typing a value to replace a formula, and so of course the formula is gone forever. However, none of the values used there are quantities in units of grams, so it's unclear what you are trying to do here. The formulas are using values from Sheet1, column C. These values are not numbers of grams. They are either text ("kg", "500g") or currency values (£0.31).
It is unclear what "Units" means in the spreadsheet. Those numbers are all just typed in.
The column "Price per piece (ex VAT)" is C*D but when column C contains text, which is almost always, the result is blank. That doesn't seem useful.
At any rate, if you need to type in values to use instead of the values provided by formulas, which is what I think you might be asking, one simple solution is to add a column such that if a number is typed there, the formula uses that instead of the default calculation. I didn't want to do the work to show you that until I was sure I had a clear understanding of your problem first.
By the way, the formulas in columns A are more complex than necessary. I would suggest this in A4 and copy down:
As a bonus, there are also some spelling errors in the Italian 
Margherita
Diavola should probably be Diavolo but not sure what you have in mind there
Quattro Formaggi
Puttanesca
Frutti Di Mare (no i)
Prosciutto
Bookmarks