If you can see that some Walgreen's greenhorn assistant doesn't seem to be familiar with requirements, leave the store and go to the next one.
Also, arm yourself with the requirements for your visa/passport/country so that you know just by looking at the results yourself in the store, that it may need to be redone. And they will usually re-do something if you point out that it's not going to work. When I had my passport renewal photos done at Walgreens, she did it over twice at no extra cost because she herself noticed that the first set was in some way not conforming to the computerized settings/requirement parameters. The technology is very good and as long as the store clerk is not completely incompetent, they should be able to comply with whatever you need -- US or UK standards. It's all in their system and the picture they take gets put through their machine and checked against the parameters for head size etc.
There is no need to leave the store with photos that don't match the requirements as long as at least one person knows what they are, and if that can be you then all the better.