Best way to keep my dad warm apart from have the heating on 24.7

An elderly person needs some heating on even with an electric throw ( the dreamland husky one is gorgeous). However maybe your DF can contribute to excess fuel bills with his pension/ benefits. My parents are in their 90’s and I’d go without food to ensure they remain warm. Anything else is immoral

Agree but...our boiler died about a fortnight ago and can't be replaced until the New Year. House is chilly although we have a raft of electric heaters, electric blankets and heated throws. Sitting in cold air isn't great. I've also had an evil cold and it made me think about how hard it would be to be elderly, vulnerable and cold.
Some sort of very low level background heat is really helpful, though I'm not sure about heater models etc.

Thermal vest
Thermal leggings
Slippers
Oldie
Snood
Hat
Gloves
Put long curtains or a blanket infant of your front & back doors on the inside of the house to stop draughts
Curtains at windows
Exercise
Hot soup, hot drinks
Hot water bottle or hot wheat bag

Does he receive winter fuel allowance ?

Wool.
Wool socks
Wool blanket
Wool hat.
It works for sheep, and it works for humans too 🥰

Warmth round the neck is good .... roll neck jumper? Start with a long sleeved thermal vest, then thermal roll neck, then fleece. Warm fleece lined trousers.
You may need to look at turning thermostat up a bit ... 21 is probably insufficient for someone who is immobile with elderly circulatory system.

I have small oil filled radiators on in the living room and bedroom. I wouldn't say they're toasty but they do take the chill off rather nicely. And I use hot water bottles a lot ~ because of this I replace them every year as they've had a lot of use. Amazon does wee half litre hot water bottles which will fit in the front pocket of an Oodie.

Heated blankets are amazing.

I agree with a heated blanket. It’s the best thing ever!

True, it still depends on what you're trying to achieve I suppose.

When I had to pop into the office during lockdown it was really cold. Heating was electric and a bit crap so no point trying to warm the whole room. I popped an oil filled radiator under the desk and it was lovely once it warmed up as it heated the space under the desk then the warm rose and warmed the rest of me.

I'm looking at ceramic heaters atm, have one in my new work and it's so small and easily portable. DS room could occasionally do with a bit of a heat boost and putting the central heating on in the whole house to heat the smallest bedroom seems overkill.

Heated blanket and plenty of warm drinks.

Yes a heated blanket and also vest and longjohns. I have worked with the elderly and I would also recommend a woollen scarf, not a bulky one and also a
small kind of beanie hat. Sounds ridiculous I know, but the head loses the most heat .
My dad would wear a beanie hat indoors and it didn’t seem odd, if that makes sense ?

That just balances out the time it takes them to get hot when you first switch them on from cold, it’s not free heat. Personally I’d rather have the heat instantly when I need it and then turn it down/off when I don’t.

Chair exercises are important too.

Oh I have that one it’s lush
I got a few of them for family for Xmas presents a few years ago and they all love them
it heats up so quickly and it’s so warm

also OP make sure he is wearing socks
I wear thick fluffy socks with grips on them
but a decent heated blanket is a must

rarely have my GCH on it’s set at 18 so kicks in a bit at night but I don’t notice as my heated blanket keeps me absolutely boiling

and a hot water bottle but he may struggle to make sure it’s locked tightly and not leak

But also warm food and try and get him moving as much as you and he can.

What is he wearing at the moment?

Themals.
Long sleeve top and long johns under clothes with thick socks

If you have circulation problems - Some Heated throws / products arent recommened

Blankets - the more the merrier.

Not electric ones, obviously.

Plug in heated pad for behind back and heated blanket. I love mine.

Heated blanket is the way to go. Don’t have to heat the whole house. I have a dreamland one - can set the timer to 1/3/9 hrs and has 6 temperature settings. I find 3 toasty. So impressed that I got them as presents for in-laws as they don’t want the heating on all the time and would struggle getting thermals on and off.

Never had a heated blanket but they do sound nice. We have one little room that sticks out from the house and the normal heating is not meaty enough and the room prone to damp as well as being uncomfy. One wall is icy to touch. So we use an oil-filled radiator on a timer to cover the hours at night when the central heating is off. Works a dream.

Heated blanket or a warm hot water bottle on his lap.

Definatly heated throw but if you can afford a goose down duvet I recommend soak and sleep v v light like a cloud

What about a Heat Genie?

Non-electric and large enough to completely wrap him up in it.

Silentnight Heat Genie Self Heating Giant Blanket - Cosy Warm Sherpa Fleece Throw with Unique Foil Layer to Retain Heat - Extra Large Reversible for Sofa Couch Bed Camping - 180x240cm, Green : Amazon.co.uk: Home & Kitchen

Thin real wool layers. It has to be real wool. Look in charity shops for merino wool jumpers etc. Layer with thermals. So many modern jumpers look warm but synthetic fabrics don’t hold the heat. It makes a big difference.