Repeat AppleScript if User Confirms a Pop-Up Dialog
First, you're making your life more difficult by using the clipboard and GUI scripting. BBEdit has a full scripting dictionary designed for working with text; I've modified your code to use it.

All you need is a repeat loop, and then a conditional exit repeat, as follows:

-- first find or create a text doc called 'Transcription.txt,' and select it
tell application "BBEdit"
    set target_doc_list to every document whose name is "Transcription.txt"
    if (count of target_doc_list) is 0 then
        set target_doc to make new text document with properties {name:"Transcription.txt"}
    else
        set target_doc to item 1 of target_doc_list
    end if
    select target_doc
end tell

-- repeat loop to run through Safari pages
repeat
    tell application "Safari"
        set page_title to (name of current tab of front window)
        set page_text to (text of current tab of front window)
        close current tab of front window without saving
    end tell
    tell application "BBEdit"
        tell window 1
            set end of contents of first text document to return &amp; return &amp; page_text &amp; return &amp; return
        end tell
    end tell

    tell application "Safari"
        set next_page_title to (name of current tab of front window)
    end tell

    set theAlertText to "Finished with page " &amp; page_title &amp; ". Next page is titled '" &amp; next_page_title &amp; "'. Continue?"
    display alert theAlertText as critical buttons {"Quit", "Process"} default button "Process"
    if (button returned of the result) is "Quit" then
        exit repeat
    end if
end repeat


This pulls the text of the current tab out of Safari and puts it into a variable, then adds that contents of that variable (padded with carriage returns) to the first document in the first BBEdit window. It then asks if you want to repeat with the next tab, exiting the loop if you say 'Quit'

You could modify this easily enough to skip over tabs, or to select the tabs to process in advance. Let me know if you want variations of that sort.