Measuring the amount of UHI there is in the environment is tricky.
The above transit in Oxford, UK may supply a site to determine is range and outcomes.
Sites A and C are slightly below the town itself in elevation, which sits on a ridge of land running down from the North. They are situated close to the edge of typical winter flooding but otherwise their local climate is driven almost entirely by evaporation requirements.
Site B is in University Parks which, although it is at the same level to the main town, itself is a green and open area. As it is at the same level as the town it should be able to sample the bulk air temperature of that elevation in non-built areas.
The world renowned Radcliffe Observatory with its long and detailed temperature and rainfall records is just below the transit line as shown in green.
It would therefore be able to provide a second, built environment, place to obtain temperature and other readings to provide a second in town sample point, D.
This is, I would have thought, an ideal place to examine experimentally to see what detailed, and potentially back-castable, temperature series in Oxford as any UHI estimated discovered can then be removed from the Climate figures.