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

Warm clothes really help.
I brought my mum an oodie and a hearted blanket.

This is a great recommendation. If he is mobile can you get a reminder to do some movement every hour? Chair exercises in YouTube maybe?
But second everyone else
Sheepskin slippers
Heated throw
Heated seat pad/booties
All on sale in Costco at the moment

I also recommend a heated blanket but take care if he has thin skin or any cuts or wounds.

Layers
Warm socks
Slippers
Dressing gown
Blankets
Hot water bottle
Hot drinks and food

basically I agree with everyone else - heat the person not the room.

Way, way back in the midst of time as a staff nurse I took the temperature of all the elderly ladies on the ward i worked on. It was the height of summer & hot and each and everyone of them was verging on hypothermic. In very large part the way we human keep ourselves warm is by moving about. If you can get your dad moving around - not always possible but if you can get him standing up and walking around every hout, even just a few paces it really helps generate warmth. If he can't stand then arm exercises can also help. Trouble is when we feel cold we sort of tighten up and stop even fidgeting so we have to be deliberate in choosing to move.

Thin wool or cashmere layers. I would recommend a high neck jumper or even a buff to stop heat escaping around the neck.

If you can get Icebreaker in the U.K., get some of their base layers. The higher the number, the better.

What everyone else said but also to add sheepskin slippers.

Thermal longsleeved top and leggings to wear under his usual clothes. If you can find a 100%merino wool set that will be better than synthetic materials claiming to be thermal.

then as other have said, hot water bottle or heated blanket to warm him rather than the room. But it might be the thermal clothes are enough to take the edge off without need a hot water bottle or heated blanket

I think its all been said but an electric throw was the best investment I made. I went with Keplin via Amazon and I use it all the time. I'd also suggest a gilet (traditional or heated) as that is a good layer that can go on top of anything and fingerless gloves. If you could get him moving more - at least once an hour that might help a bit. "Heat the person not the home".

Heated throws and hot water bottles. You can put a hot water bottle under his feet, or even heated slippers/foot muffs.

Hats keep in a lot of heat and a cosy scarf for his neck and chest area.

Warm drinks and soup help as well.

Would he wear an oodie? I have to say though a heated gilet is great. I sometimes wear one to work. OK I resemble the Marshmallow man as I have to wear it under my work fleece but it does mean you don't get cold when getting up to go to the loo or whatever
Hat and thermals especially socks.

I have a Berkshire heated throw for my chair in the living room. 4 heat settings & 4hr cut out. It's velvety soft & not too heavy. I have mobility problems & it's easy to move. Currently available in Costco for approx £32.
On my bed I have the Dreamland one. 6 heat settings & 3 timer settings. Heavier but nice & warm.

Gas central heating is cheap but is better used to warm the whole house rather than one spot where the person who needs to keep warm is.

Heated blankets I think are the most efficient in terms of keeping people warm and the costs down.

All electric radiators are 100% efficient but tend to be very expensive to run to keep people at high temperatures as they are often situated several feet away.

People seem somewhat obsessed with oil filled radiators but they are no more efficient in terms of thermal output than any other radiator.

https://amzn.eu/d/8XdC3S6

One piece oodie

Definitely a heated blanket we have one for the sofa for sitting and watching tv.

My mum has a heated pad behind her back and she loves it. A heated throw would be a tripping hazard for her as she struggles to get up.

First things first - does he have thermal vest and long johns on under his clothes, with wool socks & sheepskins slippers?

Secondly, ime 21 is just not warm enough for elderly it needs to be 22-23. If you can’t afford that does he have any savings that could contribute to the bill?

Be warned that elderly can get hypothermia very easily.

I am afraid older people do feel the cold. And do need the heating on. Cheapest heating will be gas central heating. Electricity and oil are expensive.
By all means get a heated blanket but if the air around is freezing this is not enough.
Is he claiming all he can ? In terms of pension, etc. Nobody should be cold.

I second the recommendations of a heated throw/blanket. My mum (82, always freezing for some reason even though she's quite active) loves hers and uses it every evening. I also use one when I work from home and want to avoid having the heating on all day.

I'd also say to make sure he's wearing warm, preferably sheepskin-lined slippers as for some reason having cold feet tends to make the rest of your body feel cold too.

Heated blanket
Heated foot thing
You can get electric gilet.
You could get a small Infra red panel that provides directed heat just at him.

Heated blanket and electric hot water bottle, it avoids any spillage.

Heated throw......I have the dreamland one. DP got me it years ago, everything I plug it in I say to him this makes up for all the crap presents you've got me over the years tenfold 🤣🤣🤣

There are lots of tips on this, including a comparison of costs, on Money Saving Expert linked below:

https://www.moneysavingexpert.com/utilities/heat-the-human-not-the-home-save-energy/

Feather duvet, second the heated blanket as well.

Ooh, interesting. We might try that as a spare for him, I didn't know they did that.