Most policies cover inpatient cover treatment as a minimum.
With most, if not all, private medical insurance in the UK, you'll find that inpatient treatment is covered, even in the most basic. Alongside this, day-patient treatments will also usually be covered.
Outpatient services are an optional extra.
Outpatient treatments, consultations, tests and scans, where you aren't admitted to hospital, are usually an extra, which you can opt to include on your policy for a higher monthly premium.