There are dozens of airports that can be used to get to the Greek islands, both on islands themselves and on the mainland. Not all have direct flights from the UK but most will have connecting flights from Athens if not.
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The Greek islands with direct flights from the UK are currently Corfu, Kefalonia, Zakynthos…
The flight times from the UK to Greece are around 4 hours. It can be a little under or a little over depending on where in the UK you are coming from and which island you’re heading to. For example