Heathrow transfers in July

Probably. The real question that will come up is this one reservation or two? If they are all on one ticket, then even if your arrival flight is late, you are protected and British Airways will get you to Malaga even if you miss the connecting flight. If they are separate tickets, it might be more of a challenge if the first flight is late.

Assuming it arrives on time, you have sufficient time to make this connection. When you arrive at Heathrow, follow the purple "flight connections" signs and you will do fine.

Assuming I've interpreted your date correctly as the 4th July 2019 and your inbound flight from Boston is the 09:10 BA flight to Heathrow T5 and your outbound flight is the 12:25 BA flight to Malaga also from T5 then you'll be fine, that gives you a 3 hour 15 min layover.

https://www.heathrow.com/flight-connections/personal-connection-planner/BA202/BA454/04-JUL-2019/04-JUL-2019/5/6B

Buy your ticket on ONE ticket between Boston and Malaga to be on the safe side as there is always a risk your transatlantic flight could be delayed with thunderstorms in the summer. Do not buy two separate tickets.

Thanks, all. For clarity: I have ONE reservation booked from Boston to LHR. I am considering then buying a second ticket to go from LHR to Malaga 3.5 hours later. Wondering if I'd have enough time to clear passport control, then get my bags, and check in for second flight.

The link JC provided is based on a SINGLE ticket scenario.

What you plan is the high risk way, and given you indicate you have checked luggage to retrieve and re-check, you are living on the edge.

If you can condense to eliminate the checked baggage, your odds improve significantly

To state the incredibly obvious, buying a single ticket would have been better than buying separate tickets.

Good luck

For clarity: I have ONE reservation booked from Boston to LHR. I am
considering then buying a second ticket to go from LHR to Malaga 3.5
hours later. Wondering if I'd have enough time to clear passport
control, then get my bags, and check in for second flight.

Even if you are prepared to buy new roundtrip tickets between LHR & Malaga if you miss your flight to Malaga as it could close 30 minutes earlier thereby only giving you 3 hours, tickets on future flights could be oversold.

Are you a gambler? I'm not as 3.5 hours especially in the summer is not enough IMHO.

I wouldn’t do it either. I know July 4 is irrelevant in the UK, but let’s assume some chucklehead does something stupid in the USA that day and there’s extra security in place worldwide.

If you can’t get it all on one ticket, then go to Malaga the next day or fly into LHR a day earlier. Yeah, you’d make the flight 9 out of 10 times, but you’re also going to spend 8 hours sweating it going over and that’s not the way to start a vacation, in my opinion.