There are 10 official UK cities whose names begin with the letter S :

  • Salford
  • Salisbury
  • Sheffield
  • Southampton
  • Southend-on-Sea
  • St Albans
  • Stoke-on-Trent
  • Sunderland

These are taken from the UK government’s official list of cities, which is the standard reference for what legally counts as a city in the UK.

If you broaden the question to towns and villages starting with S, the number jumps into the thousands (around 4,000 across the UK), but those are not all cities in the formal sense.