https://jobs.rcgp.org.uk/jobs/locum-sessional-gp/
A locum GP gets between £60 and £100 per hour. Which means the cost to the hiring GP is higher, because there will be agency fees and other on-costs. Whether the hiring GP sends the locum to vaccinate or does it themselves and hires a locum for regular surgery work, or simply absorbs the extra work that's the economic cost of the vaccination work.
At the very bottom end of £60 an hour ignoring on-costs, £25 is 25 minutes. Even in a town, good luck doing a vaccination via a home visit every 25 minutes allowing for travel.
Taking a more realistic approach to true economic cost, £25 is about 10 minutes of GP time.