I can understand your Parish Council's caution, however, they MIGHT be wrong in law.
As the council has GPC by default have Well-being powers. Such that they may act under Section 2, Local Government Act 2000 (as amended), to provide for the 'the promotion or improvement of the social well-being of their area" That power is used to spend on health related projects across the country.
Obviously it depends on whether it the provision of a service or capital spend.
If it is capital spend (new building / building work etc), then it is more difficult as the surgery (which is legally a private business) would make a gain on the sale of the property without benefit to the community. - Difficult to justify.
If it is to convert a disused office and the parish is paying for a nurse (or someone/something) to provide a public health benefit e.g. a new diabetic clinic in the area, then it can much more easily be argued as a public benefit and more easy to justify.