I'm not sure exactly which site recorded the 38.1°C in Cambridge yesterday, but we do know thanks to the Met Office that it occurred at 1537 BST, although I'm not sure I would describe Cambridge as being in southern England, but then again you couldn't say it's strictly in the Midlands or East Anglia either.
The location the 38.1°C occurred may have been here at the Cambridge, Botanic Gardens:
Or it may be here at the NIAB site just outside the City:
The location of the enclosure (if I've identified them correctly) at both sites is certainly better than at Heathrow, but that's not too difficult to achieve. Neither site, especially the one at the Botanic Gardens, is perfect but I suppose that's just how things are these days, and reflects how our villages, towns and cities are forever expanding and joining up.