LHR T2 is pretty efficient, and I wouldn't be particularly concerned about a connection that length - particularly with the number of flights heading to ZRH every day. Once you hit UK soil, your consumer protections increase so LX would have to route you at the earliest possible opportunity which would include moving you to a rival carrier such as BA.

In terms of timing, it's about 10-15 minutes to get from T5B to T5A - always use the elevators rather than the escalators to get down to the walkway and back up the other end, it's much quicker. Transfer security should be 5 minutes, and no gate in T2A is more than a 7 minute walk. Unless you're last off the plane, you should be at the next gate prior to boarding commencing.

If you were able to share the flight times we'd be able to give an even more nuanced answer on risk - generally speaking, Heathrow doesn't suffer any significant delays with early morning flights, so if you have a - say - 7pm departure from EWR then there's a high chance you'll actually be at gate early.