Yes, it is possible to cover the cost of physiotherapy treatment via a Private Medical Insurance policy. Depending on the policy, this may be included as standard or it may have to be added to the policy as an additional benefit.
It is common for plans to automatically include physiotherapy that relates to inpatient treatment (for example, physiotherapy following Orthopedic surgery), but standalone physiotherapy is usually an additional option for you to choose when you take out your policy.
There will usually be a limit on how much physiotherapy you are able to claim for, which is either a monetary amount (such as £500 per year) or measured by the number of sessions you are able to receive (please note this will vary from insurer to insurer).
As with all insurances, the policy is designed to cover new aliments that arise after taking out cover so it is not usually possible to claim for existing conditions / physiotherapy you are already receiving.