I would not avoid Just Eat because they do not show hygiene ratings on their website, I agree that foremost it is not the responsibility of Just Eat. I think a good way forward is a voluntary agreement by business to show their ratings. Customers can investigate further themselves if they wish to.
Hygiene ratings are not as black and white as people assume they are, when they seem them listed on the council. A lower score does not always mean that the premises is rat or cockroach infested or even being run badly. The business is marked on a range of things, not just the food itself and if an inspector happens to walk in (it is always unannounced) when a new member of staff is working that is not fully up to speed on where the training records are, the premises can be marked down by the inspector. That happened to our business, we went down by 2 points because the staff member could not point out the store handbook or training records (staff received their training online). We immediately rectified the issue with all employees, but our score had already been marked down on the council website and it took another 3 months for it to be marked back up again to 5 stars. Therefore if a business has a low rating, the customer needs to do a bit more research first before taking a decision.
Also something to bear in mind. Some councils have started to charge for revisits if a premises wants to correct a low hygiene score. Whilst a council will always reject the idea that ratings may be lowered to ensure that money comes in from a re visit, it is an easy way to get the cash in. I spoke to an Environmental officer a couple of weeks ago, who emphatically denied that charging for re visits was a tax raising exercise, but then told me in the next sentence that they were cash strapped and really short of money.
As far as allergies are concerned, I do not think that anyone with a severe allergy should purchase from any business that does not display a list of allergens. We also give strict instructions to our staff never to try to advise on allergens, but to point the customer towards the allergen list displayed in the premises. You cannot expect all staff to be up to speed on allergens and a customer should not rely on information provided by a staff member. If a customer has not been given accurate information by being unable to check an allergen list, then why take a risk and purchase the food?