How much time at Heathrow?

Be prepared to go through security again at heathrow is all I can add.

You should have plenty of time. But, as the previous poster noted, you'll have to go through TSA-type screening for your homebound flight. Best not to settle in for the layover till you're through that part.

As the others has stated you will be going through passport control and security, so depending on how many other flights have arrived its a good idea to get through both areas before determining if you have time to grab a bite to eat or shop. We have been at heathrow when huge planes have arrived and it has taken us at least 2 hours to get through the system. Once we were actually stopped and not allowed through passport control until it was less crowded, and another we missed our connection to Paris due to the crowd.

One of the main reasons we dont fly through heathrow any longer.

You should have plenty of time.Safe travels.

One ticket or two?

Transfer is T5 to T2 and the MCT on a single ticket is 90 minutes so there is more than enough time and you are protected anyway. There is no need to enter to enter the UK as you can transfer airside by bus but there will be a security screening.

Separate tickets are a bit different - without checked bags the procedure would be as above but without protection in the event of a late arrival. With checked bags you would need to collect them so it would be a land side transfer. Should still be doable but rather less margin.

I usually allow 2 hours and you have more than that. You should be fine. You don't need to rush so that will be great.

I will emphasize what Marco said. If you purchased the entire flight itinerary from one airline, you have one ticket. You will be a transfer passenger and have to go through a usually long security process, mostly because of long waits in line. When you check in at Barcelona airport, be sure your luggage is tagged through to the U.S. and that you have your boarding pass for the United flight. If you don't get the second boarding pass, then ask what to do when you alight in LHR. You may or may not have to follow the procedure below (probably will, though).

If you have separate tickets for each flight, you will probably be an arriving passenger, have to go through passport control, then wait to collect your luggage, then amble on over to the other terminal and go through standard check-in and security procedures.

Heathrow's website has a very detailed connection tool. Put in all your details, and it will tell you, step by step, exactly what's involved and how long it will take (times, of course, are estimates, not guarantees): http://www.heathrowairport.com/heathrow-airport-guide/flight-connections/connection-guides

I have made that transition myself on occasion and four hours should be fine, although much of that will depends on conditions beyond your control, as noted above.