That was your mistake. Lebara is a shit network aimed at newly arrived immigrants, because it offers cheap calls to Poland, India, Pakistan, China, Afghanistan, etc. Using them for data and you're gonna have a bad time.
What you should have done is pick up a prepaid £15 sim from Three network. That would have given you something like 250 minutes, 5000 texts, and unlimited 4G data for 30 days.
There's 4 major providers in the UK - O2, Vodafone, EE (combined Orange & T-Mobile) and Three. Any other networks are just virtual networks that hire cell towers and bandwidth from one of the 4 majors.
As far as using my Three SIM for free data in the USA, as far as I can tell, the speeds are 3G speeds rather than 4G. But that's cool with me. Last time I was there, I had to buy a pre-pay SIM that cost something like $45 and it only had 500mb of data which I promptly burned through in about 3 days.