Personally, I’m going to supplement with Ultima Replenisher Electrolyte Hydration Powder, a relatively cheap and easy find I looked up on Amazon.
Generally, look for electrolyte mixes and such with sodium, magnesium, and potassium. Extra stuff is nice but not necessary. If you’re feeling faint or breathless during the day, drink water with dissolved Himalayan salt slowly, and you should feel better. I’m still feeling this out - just like you - but as I learn from my mistakes and do more research, this is what I have found to be useful.
It doesn’t have to be straight water! Tea, (low cal) sports drinks, etc. all count. I bought some electrolyte powder on Amazon and I’ve been liking it so far. Kind of sour compared to Gatorade, but way more flavorful than plain water. I also quite enjoy Mio Sport which comes in zero calorie. I think Walmart has Mio?
Ultima Replenisher Electrolyte Hydration Powder, Variety Pack, 20 Count Stickpacks - Sugar Free, 0 Calories, 0 Carbs - Gluten-Free, Keto, Non-GMO, Vegan https://www.amazon.com/dp/B01EGP1KYY/ref=cm_sw_r_cp_api_i_bfAeCb6G97DNP
The Mio blackberry raspberry and tropical cherry flavors are sweetened with stevia and don’t have artificial sweeteners. They’re not my favorite, but they’re okay and a lot of stores carry them. I also like Ultima electrolyte mix. I buy it from Whole Foods Market. Half a packet is enough for 16 oz of water for me, I don’t like it full strength except in yogurt. https://www.amazon.com/Ultima-Replenisher-Electrolyte-Formula-Stickpacks/dp/B01EGP1KYY
You could be low on electrolytes. I'd start there. I really like Ultima's product.
I may have, but don't recall it specifically. I had a zero carb electrolyte pack in my bag for a while, but have since not replaced it. It was something like This but I don't recally the exact brand I had floating around.
I'm 5'2", and yes, it's harrrrrd. I'd try incorporating the other suggestions into your meal plan first, if you haven't already.
Beyond that, I buy these. They are pricey, but they work for me, so I wanted to share: https://www.amazon.com/Ultima-Replenisher-Electrolyte-Formula-Stickpacks/dp/B01EGP1KYY
They would be completely portable for your trip (I'm just not sure about U.K. availability, which is a wild guess at where you are). I have an 18 oz water bottle I carry everywhere and I use one packet in each fill.
Each serving has 100mg mag, 250mg pot, 55mg sod, no cals. Low on sodium which I know is a struggle for you, but I personally never have trouble hitting that one.
Edit: Similar to mio, but powdered. Tastes like koolaid.
It was Ultima, though I think quite a few brands have magnesium, so you have some options. I hope it works out better for you than it did for me. :) https://www.amazon.com/Ultima-Replenisher-Electrolyte-Hydration-Stickpacks/dp/B01EGP1KYY
I cook with more salt. She also drinks a couple of these a day: https://www.amazon.com/dp/B01EGP1KYY/ref=cm_sw_r_cp_apa_fabc_X5Z8Fb3Y1DGHR
She uses the tub format cause price, but I like the variety flavors. It's basically sugar free Gatorade.
I’m taking this. It’s easy to dissolve in a water bottle every morning!
I use these; zero sugar, zero carb:
Ultima Replenisher Electrolyte Powder, New Formula Variety Pack, 20 Count Stickpacks https://www.amazon.com/dp/B01EGP1KYY/ref=cm_sw_r_cp_apa_rJIxzbRQKRRYQ
Edit: I didn't answer your other questions... From what I've gathered, it's best to do your work out on a completely empty fasted stomach and then replenish yourself afterwards when your muscles are prepared for nutrient uptake. I would also say that its better to get your micronutrients and electrolytes from/with food all the time. I happen to like my meats salty so that's never an issue for me.