As stated on the GOV.UK website, being a resident of Spain, as well as a US visa holder, you do not need a transit visa. Just be sure to present your US visa at the bag drop and the gate, which you have to anyway.
To clarify, you can transit the UK without a visa either with a US visa or Spanish residence permit. You have both!
If you have two separate bookings, you'll need to cross the UK border to pick up and re-check the luggage. In this case, present your passport (open at the page with the US visa), residence permit, e-ticket of the connecting flight and filled-out landing card (which airline crew will distribute - but only take one if you're going to enter the UK)
If an ignorant bag drop or gate agent tries to deny you boarding without a visa, insist that they check Timatic, a database which has all info about these things. In that case, make sure to tell the agent to put in "Alien resident of"=Spain, and the UK as the transit, NOT destination, country.
As for Brexit, literally no one knows - they haven't even started the negotations yet.