Fuck me there's a lot here. I travel a LONG way, a LOT ... here's my 2c.
Insurance (Travel/health = ex (due to be released from out patient 
)-heart patient)
Elaborate please, who was a heart patient? First step is do you have a proper credit card like a VISA Infinity with proper insurance?
Visa's
This is easy, look up what you may need and apply, UK will be simple but check well ahead for Sri Lanka.
When to book
Now.
Car hire (UK from Can)
Autoeurope, no question.
Travellers checks (as well as cards)
WHAT? Jesus H christ, all you need are credit cards and debit cards. It's not 1991.
local Sim for mobile or get international
If staying more than a week always go local and get it from the airport when you land. Don't forget the little pin thingy to access your SIM compartment.
Stop watching youtube videos on subjects like "disaster in the air" , "why flight 123 crashed without a trace"
For someone who travels so much and so far so often, I still feel uneasy the entire way but just look past it. Remember, seat belt ALWAYS on in case you hit turbulance.
Travel adaptors
You will need them so buy them. Lots of them if family has cell phone or multiple devices.
Laptops/devices to take
One per person, absolute max.
Dont drink the local water 
UK will be fine, bottled only in Asia outside of Japan/Korea
Haven't traveled in like 12 years, feels a bit daunting ! Especially as a responsible adult and not a happy go lucky single person
OK so a few other things. Paying business/first it utterly INSANE. I don't care what you and the mrs make, it is INSANE. Grind it out princess, put the money away for the kids deposit on a house. 35K is absolutely mind-boggling nuts. Harden the fuck up.
I highly recommend a proper travel credit card like the Infinity ones mentioned, they have excellent cancellation protection and insurance though there are length limits so you may need to add a little insurance if travelling longer than the coverage period. They also have excellent car insurance but you MUST A) book with your card and b) reject all insurance from the rental agency.
Read ALL THE FUCKING FINE PRINT on pre-existing conditions as you may need supplementary coverage. You touched on a heart issue for someone in your party, if you have that on record and you need a hospital visit on anything heart related you may not be covered so make sure you are or you risk being fucked super hard.