Commentary

English place names would be so much easier to pronounce if educated people in the Renaissance and the Enlightenment could have resisted the temptation to tinker with them. 'Thames' has never, ever, been pronounced with a 'th-' sound. In pre-Roman times it was Tamesas, from a Celtic word meaning 'dark'; the Romans called it or Tamesa; in Middle English it was spelt variously Temese or Temse; Sir Thomas Wyatt in the 1530s spelt it Temis. But at some time in the Renaissance some over-educated prat produced the theory that it was the same name as the Some Renaissance scholars believed that the river was named after the River Thyamis in the Epirus region of Greece, and that the name had been brought by Celtic tribes migrating from there to England; and so they stuck an H in it. But it has never been pronounced.

'London' is another. In fact the first vowel sound was originally O: Greek and Roman writers spelt it Londinium and Londinion. But by the 4th century Ammianus Marcellinus was already spelling it Lundin(i)um, and the change of vowel sound was permanent. From then on through the Saxon period into Middle English it's Lundenburg, Lundenne, Lundenceaster and just plain Lundin all the way; till Gothic scripts composed of straight vertical strokes came into fashion. In these scripts the sequences -un-, -um- and -uv- are almost impossible to make out, so scribes took to replacing a short u before n, m, or v with an o. (Hence the modern spellings of the words honey, come, and love.) So the usual spelling accidentally reverted to the classical form, which hadn't been pronounced for near on a millennium!

And I used to live in the London parish of St Marylebone, which was originally St Maryborne or Marybourne, i.e. 'St Mary['s church by the] bourne' ('bourne' means 'stream' - in this case the Tyburn stream, as it happens). But somebody in the 18th century got the notion that it had originally been 'St Mary la bonne' and started to spell it that way. (FWIW, locals pronounce it Marleyb'n, more or less.)