According to ONS statistics released 24 May 2019:

• Overseas residents made 37.9M visits to the UK in 2018, 3% fewer than in 2017 - the first drop in visitation since 2010.
• Overseas residents spent 22.9B GBP on visits to the UK in 2018, down 7% from 2017.
• Average length of visit in 2018 was 7.0 nights, down from 7.3 nights in 2017 - likely a factor in reduced spend.
• Distribution of reasons for visits to the UK in 2018:
  • Holiday 15.1M
  • VFR 11.8M
  • Business 8.4M
  • Other 2.6M
• Country of residence of visitors to the UK in 2018 (top 10):
  1. USA
  2. France
  3. Germany
  4. Ireland (Republic)
  5. Spain
  6. Netherlands
  7. Poland
  8. Italy
  9. Belgium
  10. Australia
• UK cities visited (top 5)
  1. London
  2. Edinburgh
  3. Manchester
  4. Birmingham
  5. Glasgow

Uncertainty related to Brexit likely accounts for a good part of the dropoff in visitation from EU states (and thus for #1 2018 source market being USA).