It’s actually not as clear cut as the Spanish economy tanking because the Brits are shut out because there are a couple of other factors at play.
One is that in some places there there is a shortage of accommodation to match seats available, so an element of market readjustment and an element of other countries and operators taking on capacity that becomes available.
Another is that various destinations are looking to clean up their act and attract higher yielding tourists. The Spanish Government might secretly welcome British mass tourism falling off a cliff.
If our economy tanks as everyone expects it to even if barriers are temporary we are still going to end up with a much longer term drop in demand.