For the following example:
I had seen many beautiful shops in UK, when I was walking down the street years ago.
I'm not sure if the phrase "years ago" at the end of this sentence is correct. Should I put it before the start of the sentence?
I'm not sure whether you're asking about the use of the Past Perfect (Had P.P.) or just the position of "years ago" in this question.
It seems to me that the matter of how the Had P.P. was applied is more important.
It would be enough to just say:
"I SAW many beautiful shops in the UK when I was walking down its streets years ago."
The "Had P.P." signals an action that happened even earlier than an already past action. So, your example above could mean:
"I HAD SEEN many beautiful shops in the UK when I was walking down the streets (of Cairo) years ago. So I didn't appreciate Cairo very much."
But of course if you added the Past Action differently in your context, your Had P.P. would be right.
e.g. I didn't enjoy Cairo very much. I HAD SEEN many beautiful shops in the UK when I was walking down its (UK) streets years ago."
Your should add a "the" before UK. Also, change street to streets, unless you are referring to a particular street. You could also try:
And I am sure you can think of many such alternatives. (You many consider omitting/replacing one "many" from some sentences above as it sounds awkward - with two "many" close by.)