55 minutes is below the MCT for T1 to T1 international transfer. You shouldn't be allowed to book it as a single ticket. And booking it as separate tickets would be insane as neither United or El Al would take any responsibility if you missed your onward flight.

3 hours is a good time IF you can get your luggage checked through, but on a bad day at LHR you could easily spend half that just getting through immigration & collecting luggage, let alone check-in and going through security which would make a late arrival a problem.

I'd also check how long in advance of the flight El Al require you to be at check-in. They do ask a lot more questions & do a lot more checking than other airlines before even issuing boarding cards.