Transit service?

Because of your separate tickets, you would need to meet all of the requirements for entering Spain. Which you would do anyhow if you were flying onwards within Schengen.

There is a security checkpoint before passport control if you are staying outside of Schengen, but I don't recall it being operational the last time I arrived into BCN from LHR... You definitely would need to have your VY boarding pass, which isn't necessarily guaranteed online due again to COVID restrictions...

Worst case scenario is you have to pass through passport control and the health checkpoint, then rush upstairs to come back through security.