I think that it is you who does not understand it really...
Using your example 12 month ago £250 would buy you that lens or say pay your utilities bill for the month or pay for a 2 week worth food shopping (for example). Now, today, the £400 will get you the same lens, pay your utility bills for the month or pay for a 2 week worth food shopping. So if I bought my lens 12 month ago for £250 and sell it today for £400 - what exactly am I gaining in terms of value????? It would be a profit if I go 12 months back in time and can use it then at those values/prices but not now.
What you are confusing again is the value of things and its monetary expression - value changes somewhat but has no relation to money (it is always defined by demand/market, includes depreciation if used etc). Monetary expression of the value is different at a different times and in different countries - is it really so difficult to grasp? What you would buy with 10000 pounds 20 years ago is nowhere near equivalent to what you can buy with the same amount today (you could have bought much more for that amount back then) so why you expect the price expression of the value to stay the same over time??