The poem
Lonely in Ireland, since it was not home, Strangeness made sense. The salt rebuff of speech, Insisting so on difference, made me welcome: Once that was recognised, we were in touch
Their draughty streets, end-on to hills, the faint Archaic smell of dockland, like a stable, The herring-hawker's cry, dwindling, went To prove me separate, not unworkable.
Living in England has no such excuse: These are my customs and establishments It would be much more serious to refuse. Here no elsewhere underwrites my existence.
Larkin had issues with belonging. In 'For Sidney Bechet' he comes closest to an environment or emotion that makes him feel at home.
This poem is about Larkin's preoccupation with the reason he cannot feel at home and the guilt he has about feeling strange and out of place.
He is 'Lonely in Ireland, since it is not home,' and yet he was made welcome because it made sense to feel strange. These were not his 'customs and establishments'. At last he had an excuse to be out of place. He may not be entirely comfortable but 'Here no elsewhere underwrites my existence.' He can be strange here if he wants.