Perennial underplanting ideas for large potted olive trees in the UK

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We have 3, large olive trees which we are re-potting into even larger pots which are about 80cm across. What flowers/plants could grow around the stem? Ideally perennials and maybe some that hang over the edge.

Maybe Erigeron karvinskianus 'Profusion’, little daisy like flowers pink & white, bloom for ages and they self seeded into my pots under citrus trees & are quite happy.

Thyme.