Well, experts or not, boundary layer or not, the 12z GFS paints a still-remarkable picture in the south at 850hPa, but a much less remarkable one down at ground level.
Saturday still looks to be the peak of it, but it's a short window of opportunity. I also notice that the silly high dew points are now entirely absent - no more 27s and 28s, but that shouldn't be a surprise to anyone considering that a) temperatures are generally forecast to be lower than that and b) those would have been 3 or 4C higher than the record UK dewpoint.
Friday in the SW and Saturday more generally in the south are still shown as having high dewpoints, but that's a long way out and still very much subject to change.
The 12z MetO, meanwhile, shows the 20C 850 isotherm not even reaching the SE of the UK, instead only affecting the SW. Again, there is potential there for some really high temperatures IF (and it's a big if) a foehn-type effect were to set up in the lee of the hills, but that's not presently shown by the model.
The German ICON run is the most bullish regarding the extent of the plume, but it's not supported by the major runs out so far.