Christ on a cracker. Look at these uppers.

I think it was last year when Cornwall got above 23C uppers, ironically not during the hot spell but during another less significant heatwave. From what I recall the humdity was also very high and the temperatures were not that impressive at all. Around 30C from what I remember.

All here:

https://www.theweatheroutlook.com/twodata/gefs.aspx

Upgraded today from 1 degree to 0.5 degree datasets.

Here you go

25C briefly last year which to my knowledge is a record.

But like I say temperatures at the surface wern't that impressive. The air was very wet.

2m temps around 22C!

Tbf I think they did underestimate and got to around 28-30C.

But still. With a regular sort of lapse rate an 850hpa of 25C corresponds to a upper temp pushing 38C possibly even 40C.

So given that 25C 850s were possible, if the plume had been drier and pointed at the SE during the middle of the day (rather than early in the morning) with no cloud/moisture then I reckon 40C+ could have happened soil moisture permitting.

Totally off topic, but 14 degrees is t-shirt weather in my neck of the woods. 20 is hot, mid 20s roasting!  

Its the persistence of the wind up here of late that has caused many an evening chill when sitting out. Kept the midges at bay though!

This Friday is touted to be 26 up here but blink and its gone. Will feel very hot though given its such a rise from the preceeding days.

 LLTNP 105m asl 

We're rather offf topic but that's not correct: some of the sun's radiation does warm the air directly through the water vapour, dust particles and so on.  Of course a lot of it is the long wave radiation that bounces back from the surface and is absorbed, particularly by CO2 molecules.

Arpege now going for 36c on Friday. Certainly possible with the predicted 850s. All depends on cloud cover if we can get a mainly clearer day then it could happen.

I see your link to the GEFS Brian but without it seems impossible to find?

I reckon it only reached about 14 degrees at best here yesterday but it sure didn't feel like t-shirt weather to me!

34'c would set a new date record

Not sure what you mean? 

The correct link is:

https://www.theweatheroutlook.com/twodata/gefs.aspx

All charts can be selected from that page.

E.g. London:

https://www.theweatheroutlook.com/twodata/gefs.aspx?run=na&lg=850&lglocation=london&charthour=006&chartname=ps_500hpa&chartregion=na-region&p=1&charttag=PS%20500hPa%20(GPDM)

I think you're talking about the environmental lapse rate aka temperature profile, for which there isn't really a 'regular' one- it's the vertical thermal profile and changes frequently according to changes in air mass, inversion set-ups etc.

The more 'regular' LRs are the dry and saturated adiabatic lapse rates which do have approximate regular temperature changes with altitude, but these relate to so-called 'parcels' of air rising or sinking and are not a comparative measure for surface and upper air values.

At least, I think I remember correctly. Nonetheless, the potential for the high temperatures you mention could be there.

To models, and I'm looking forward to the restoration of the regular GEFS link: when the outlook is less than summery in the Op, it gives a little flicker of hope.