Ratings are given based on 3 things. How safely food is handled, what businesses do to keep food safe ie, temperature records, cleaning schedules etc and the structure of the buildings etc. All should have 4* in my view as that’s just basic practices and procedures. The buildings part is what annoys most chefs as that’s out of their hands and up to managers/owners. So chefs can be producing the safest food in the cleanest kitchen but because some wall tiles are cracked or ceiling is in need of repair they will get a 4 instead of 5.
In my opinion those with a 1 should be closed immediately but the inspector to close a business has to be sure there is imminent risk to human health.
Inspections are carried out unannounced at any reasonable time and within a 2 year time scale. This length of time depends on your rating and type of establishment so a 4/5* place will be left for almost 2 years, a 3 might be seen again in 12to18 months. High risk places like hospitals, schools, nursing homes will be inspected more frequently too.
This all depends on staffing too as like all nowadays staffing levels have been cut so less inspectors to go round!!
If not happy with a rating you can pay for re inspection to carry out improvements, in this case you are ‘supposed’ to display rating given not an old one until re inspection. So if you had a 5 then got 4 and want a 5 again you can’t leave the old 5 up until re inspection the 4 should be put up. Most food producers I know are keen to get good ratings however most customers don’t take much notice of them ie research a rating before eating out or buying food out.