1. I'm guessing you're on a through ticket LON-EWR-BOS so your bags will be booked all the way thru to BOS.
2. Arrive Newark Terminal B (International), enter USA via immigration checks.
3. Collect baggage, clear customs, drop bags at Connections, reclear Security.
4. United Express could leave from neighbouring A or C terminal, linked by shuttle bus.
(unless its changed since my last transit)