I gather since we left the EU airlines and its crews/staff could not easily ''cross work'' amongst the old existing Bases here in the UK and in the EU (which includes Eire)
Thus we saw various airlines including Easyjet and Ryanair re register a mass of their fleets in Austria, Malta, Poland, the UK and Eire etc,
plus basing what were once UK aircraft now overseas. Both companies set up EU divisions.
This year TUI has set up Fly4, a joint venture for ACMI/sub charter flying with Enter Air. Headquartered in Ireland.
It's all a convoluted deal to get around the EU workplace and EASA rules.
I doubt Iberia could now set up a LHR base, likewise nor could BA at say, MAD.