International Passenger Survey (IPS) 2017 (2018).

The summary reports from ONS and VB give the highlights of the IPS results for 2017 (still subject to possible revision). The ONS report gives both inbound and outbound UK travel statistics, the VB version features inbound travel only but gives both UK and home nation breakdowns.  For a the full strategic overview both short summaries should be read:

2018 ONS Travel trends 2017

2018 VB UK and Regional IPS Summary 2017