In message , Basil Jet
writes

I always assumed that it was either Eye-zl-dn or Eye-yl-dn, but apparently
Isledon was the old name for Islington. This makes me think it must be
Iz-ler-dn, since that's the only pronunciation which could conceivably drift
into Islington.

The Anglo-Saxon name was apparently Giseldone (Gisla's Hill). IIRC, g
before i was pronounced as y, and the i in this position was long, like
the i in "machine". So Yee-zl-dn (and later Ee-zl-dn) was probably the
original pronunciation.

The disappearance of the initial y sound was very common: Gyppeswic
(pronounced Yipswich) became Ipswich, for example. The i probably became
short during the 15th century, as part of the 'great vowel shift' -
although one can always find exceptions to vowel shifts (such as
Gistelesworde, now Isleworth, the i sound here mutating into a
dipthong).

Having said all that, I don't know how the locals now pronounce Isledon!

--
Paul Terry