Fella, you should be inserting a cavity tray for which there are propriety products. Your inquiring about a cover flashing, which may or may not prevent water ingress at the abuttment. If your going to do it, do it properly.
Ubiflex ...just another load of rubbish. Why anyone would bother beats me. If you cant be aresd to do it properly, just throw some mortar at it. That at least will last a few years and is cheap.Unlike the spammed junk.
install making a £500.00 job cost £250.00 Lead done right will out last any other product, lead will only split if it is fitted in long lenghts with no give for movement!
no need for lead with a pitched fibreglass roof. Dress the fibreglass up the upstand and into the chase. cement/ lead/ metal drip to taste.