Alsacien,
I would agree that some cheap helmets may have inferior comfort linings etc. But this is not a market where there is an automatic price/quality link. Unless you have independent test results, you are relying on the word of the marketing department of the expensive helmet brand for their claims of additional tests over and above the standard ones. After all, how many different helmets have you personally crashed in in order to be certain the claims are not bunkum? Very few I hope! I have crash tested a small number of helmets (not intentionally) over the years and the cheap ones worked as well as the expensive ones - perfectly well in every case! Added to which, a helmet has a relatively short useful life (maybe around 5 years max for a well cared for helmet) before the more important impact absorbing core has lost its efficiency (the core shrinks gradually but continuously due to various environmental effects) and technology moves forward so it may actually be beeter to own a cheaper helmet and replace it more often. This wil be particularly true if you are careless with helmets but reluctant to replace something expensive that has only suffered a small impact. If I want to be absolutely certain that a helmet is safe, I check that it has passed a variety of other tests such as SNELL/DOT as well as being E approved. Each test is limited in its own way but they are all different and, in combination, indicate a helmet that has been designed well rather than designed to pass a specific test. But generally, for everyday riding, I dont worry about it that much for everyday use. I wear a helmet more for considerations of comfort and to comply with the law!
Based on the general quality/price ratio of LIDL goods, a £40 LIDL helmet might be expected to measure up to a helmet costing anything up to around £80 at a normal retail outlet. It will almost certainly be TUV certified as well as E certified and while not the hight of comfort or style, will be adequate for the more modest user. Not everyone can afford a £300 Arai.
On a more general note, LIDL bike gear almost always sells out quickly, ditto for ALDI, because the quality/price ratio is very good. It may not be the absolute best but then, when on 4 wheels, I can no longer afford to drive a Mercedes either!