Without compromising your anonymity SD which part of the U.K. (approximately ) are you in? Is it going to be within 40C area?
I am 60-70 miles north of London and here, highs of 39 and 37 are forecast.
I imagine you won’t be planning on going out, so the advice being given on skin protection doesn’t apply, but Age Concern advise the following
How to avoid dehydration
Make sure you’re drinking plenty of fluids. Aim to drink 6 – 8 glasses of liquid a day, and more if it’s hot.
Eat a balanced diet to help your body replace any salt you lose by sweating.
Symptoms of overheating
Extreme heat and dry conditions can cause you to dehydrate and your body to overheat.
Watch out for certain signs – particularly for muscle cramps in your arms, legs or stomach, mild confusion, weakness or sleep problems. If you have any of these, rest in a cool place and drink plenty of fluids. Seek medical advice if your symptoms persist or worsen.
Symptoms of heat exhaustion
The symptoms of heat exhaustion include headaches, dizziness, nausea or vomiting, intense thirst, heavy sweating and a fast pulse.
Symptoms of heatstroke
The symptoms of heatstroke include:
confusion
disorientation
seizures
loss of consciousness.
What to do
Heatstroke is a life-threatening condition. If you or someone else shows symptoms:
call 999 immediately, or 112 if you are in the European Union (you can call 112 from a mobile for free)
if you have a community alarm, press the button on your pendant to call for help
while waiting for the ambulance, follow the advice given for heat exhaustion but do not try to give fluids to anyone who is unconscious.
What to do
If you have any of these symptoms you must, if at all possible:
find a cool place and loosen tight clothes
drink plenty of water or fruit juice
sponge yourself with cool water or have a cool shower.
Your symptoms should improve within 30 minutes. If you're feeling better but still have any concerns, call your doctor or NHS 111 for advice.
With adequate preparation - loose light clothing, open windows (but closed curtains on the sunny side), extra fluids, perhaps damp towels, a bowl of cold water for your feet, and only gentle slow movements round the house - yes, by all means lie on your bed if that is comfortable for you, you should not be at serious risk.
There will be loads of useful advice on the NHS website I expect, Age U.K. and others.