1)「こんなに見{み}えなければイギリスではない。」 (Your original)
2)「こんな風{ふう}に見えなければイギリスではない。」 (Corrected)
Between those two sentences, 2) is considerably better than 1) if you want to say:
"If it doesn't look like this, it's not Britain."
「風に」 can be replaced by 「感{かん}じに」 with no problems.
Is 1) totally out of the question, then? I would say no because very informally, some native speakers might say it, too. But still, it would not be something you should be taught to say as a Japanese-learner.
I would like to stress, however, that in the particular situation/context of showing a person a photo, there would be little misunderstanding even if it were 1) that you actually uttered.
As a stand-alone sentence without further context, however, 1) could sound fairly ambiguous.
Finally, I have no intention of changing the purpose of your question, but if I could express the idea in question freely as a native Japanese speaker, I would not even use 「見える」, to be honest with you. You used it simply because you had a sentence in mind in English that used "to look". Your sentence was translated; Mine will come directly from a Japanese-speaking brain without translation.
I would use 「なる」 and say something like:
「こんな風になっていなければイギリスではありません。」 or
「こんな風にならなければイギリスではありません。」