Can be done easily with a few basic cli utilities. Although this wouldn't be the nicest solution to update/maintain. If your problem is actually any more complicated than this, or will need to be updated in future I'd recommend you use a high level scripting language like Ruby or Python instead.
Assuming a filename of block.text you can extract the lines you care about:
sed '/\(<SUBEND\|SUBSCRIBERIDENTIFIER\|SUBSCRIPTION\)/!d' block.txt |\
Truncate the Ampersands
cut -f1 -d"&" |\
Delete newlines, tabs, etc
tr '\t\r\n' ' ' |\
Replace the <SUBEND with newlines
sed 's/<SUBEND /\n/g' |\
Tidy up leading whitespace and doublespaces
sed 's/^ \+//g' |\
sed 's/ \+/ /g'
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