It looks like next week the UK will have a couple of days in the mid to high 30c. This is very unusual and it would seem sensible to skip those days. I seem to remember @Jonathan saying if you miss workouts you should add a note to the calendar and this helps Adaptive Training adapt. I can’t remember exactly what he said.
What should I do to the plan for those days?
personally? I would ensure I have the fans for it, and continue to ride. If its way too hot, maybe turn them into Z2 rides and go for the heat adaptations! But to me it seems like a great way to get some heat training in, safely from the house.
I usually do my workouts at lunch time, as I’ve woken up then and my body is responding to the desire to move, where I am in the UK it’s going to 38+ Monday Tuesday, and I planning to juggle my workouts so Monday is a rest day, and do my workouts in the morning where it is predicted to be sub 25 until 9am, else I won’t ride , I’m not used to that sort of temp, and when I was in Arizona, the locals who were used to that sort of temp would get up early to ride
38 deg is hot for the UK, but plenty of people from the UK go to Spain, Italy or Greece to cycle in high 30’s all week. Leave it until the evening and it will be fine to ride
You can keep spinning your legs and not worry about power or intervals. Those first days of heat I’ll dial the power way back in order to reduce the heat stress on body. Or if the heat doesn’t appear to bother you too much, you might do some endurance and shorter intervals.
Put your feet up and have a beer. 
Won’t make a jot of difference in the long run.
This is actually really good advice, especially if not got any close upcoming events… schedule few days rest as heat won’t last or just easy rides… Saying that I’m in UK and have 17 mile run tomorrow (marathon training!)… gonna be hot!
Different when hotter temps are normal and body adapts (or have no choice in long term) but UK…few days off/easy likely not bad idea at all
I live where it’s been 38+ for a week, with no end in sight. We ride outdoors as soon as the sun rises and finish up by 10am. If you can only ride inside and don’t have AC, I’d do Z2 rides with the fan on.
I would just ride them. I think it would be a good experiment in heat tolerance.
After a couple of months riding in TX I am pretty much immune to the heat. Last outdoor ride was an average temp of 104F/40C. 
I am going to have to wear a jacket when temps get back into the 80F/25C range. 
On one hand, it’s funny how the British media absolutely panic if the weather is anything other than moderate.
On the other hand, those living in hotter climates need to appreciate the massive jump in temperature for a few days only - yesterday, it was 19 C, the forecast is for low 20s today, than rising to nearly 40 C on Monday/Tuesday, before going back down to 20 C on Wednesday.
Ride early, ride easy, or go for a swim instead!
Its not funny at all.
Its the simple fact that our workplaces and homes are designed to keep the heat in and not out. Guaranteed that there will be a number of elderly people die. They are used to keeping many layers of clothing on to keep warm and not keeping cool.
Or enjoy the heat and ride outside 
Worlds only gonna get hotter - go for a z2 ride in the early morning or evening. Enjoy the sun, long days and heat while it lasts! Plus I find the sun more the killer compared to the heat. I use to run at night in dubai in the summer - mid 40s. I actually found it fine. And I wasn’t too adapted to it as I was only there 1 month a year
So, from my own experience, our summer in NZ was hitting the 30 mid day. This forced me to get into the routine of getting up early at 4:30 and getting training done then, with 2 fans and another on backup, all three tied to a single remote (you can get a 3 pack of plugs with one remote from most electronic hardware stores). Night rime was another option but not so often.
Quite. Over 70,000 people died due to the 2003 European heatwave.
I did 38 miles today for fun. The three training days were done. It was 27c and I was fine. Saying that, that is not 38c and we Brits are not used to that. We will probably moan it is cold when it gets back to a normal 23c 
When it is <5c in the winter when I train in the summerhouse, I will wish it was 38c again
Where I live (Cambridge) we have a set of rowing races all of next week and they cancelled Monday and Tuesday. So disappointing, but they were concerned about older masters and unfit participants (it’s a community event). I get on well in hot temperatures and I was looking forward to some good heat adaptation.
I assume the trains become a shitshow when it gets really hot* over there? I’m flying in Tuesday. 
*What we’d call warm in AU.
You assume correctly…if it’s too hot, too cold, too windy…etc 
Almost certainly there will be buckled tracks etc.
The thing that most of the hot weather world won’t realise is that the current maximum ever recorded in the UK is 38.7 in Cambridge a few years ago. If we hit >40, that’s a huge jump.
The worst bit about it is going to be recovery - it’s supposed to stay >25 overnight Monday, and that’s too warm for me to be able to sleep properly (even assuming the house cools down - it’s been over 23 degrees inside while cooling to 15 deg outside the last couple of nights).