21st marked the first heatwave last year, incidentally.

It certainly does look massively cooler than they were talking about. 22-23C looks about the tops this weekend. The MO extended outlook still mentions the possibility of hot conditions later on though. I am still pretty confident we will see 30C this month.

It may be gone by the time you wake up, but my iPhone app for my location has 35C a week on Sunday. Never seen that sort of temperature being shown at such a lead time.

There was a 32C for me first thing this morning (I was up at 3.30am, dad taxi service) 

Now moderated to 28C.  GFS manages four days of 30C+, two and two with a cooler interlude in between.  The AIGFS is much less warm, and the ECM AIFS is  also not convinced, with 21C next Sunday, generally low 20s with the odd day reaching the mid 20s.

GFS Op 0z - agrees with FAX and  by Thu 18th goes on to show the LP responsible for the troughs taking up residence off NW Scotland with HP intensifying over Germany bringing up increasingly warm air from the south. BY Monday 22nd, with HP 1025mb established over Britain, conditions look hot for England but cooler in the far north. Thu 25th shows the HP further north, with warmth spreading to Scotland but LP from Spain trying to cross the Channel into England. On Sun 28th the HP withdraws to the Atlantic and any hot spell breaks down as winds switch to the NW.

ECM - similar to GFS though perhaps less hot as the feed of warmth is more from the SW rather than directly from the south. EDIT after 0900 download. However, on Thu 25th ECM differs from GFS by moving the HP cell to the N Sea and bringing heat from the Continent for all through to Sat 27th with LP, not HP, in the Atlantic.

AIFS - London, temps mostly 20-21C but 25C Fri/Sat 19th/20th and again Wed/Thu 24th/25th. A little rain next week from time to time, more definitely Fri 26th. Edinburgh, maxima 15-17C to Sun 21st then rising to 23C Wed 24th before dropping back. Some rain in the coming week esp Fri 19th then dry until more on Fri 26th

GEFS - Mean temp dropping back to norm after a warm weekend to come, then rising to 7 or 8C above norm in S England falling away to 2 or 3C above norm in Scotland, lasting to Mon 22nd with quite good ens agreement. The mean then drops back to norm along with most ens members but a significant number in the S including the op run continue hot. Chances of rain from Fri 19th in S England, again with a progression northwards to a higher chance and starting  Wed 17th in Scotland.

A week of hotter weather for S England, but only a week, and not really getting as far north as Scotland. However, it looks very hot over the Continent, and it wouldn't take much to much to shift that northwards to satisfy the heat hounds. EDIT (after ECM 0900 download) and indeed ECM does just that

https://wetterzentrale.de/en/topkarten.php?map=16&model=gfs&var=17&run=6&time=228&lid=OP&h=0&mv=0&tr=3#mapref