I have another plumeria grown from a seed which is doing well after 1.5 years of sitting on this south east facing windowsil which is usually fairly warm with quite a bit of sunlight through the day.
So I'm fairly confident these cuttings should have a good chance, but am thinking its probably too late into the summer to start them now.

I don't know how useful my tips can be since I live I south tx usa. Cut wound is dried up (which is what you want ) so just put some very fluffy garden soil in a pot, stick you cutting in it and water thoroughly. Place in the absolute sunniest and hottest place in your garden and DO NOT water again until you see a leaf. That is what I do here. I worry that UK might be too cool, unless you have a hothouse. Typically, it takes me 3-4 weeks to have a rooted cutting. As foliage develops I water but very sparingly. The following year and subsequently, I water like any other tropical plant

Your potted plumie is very pretty and healthy but with a pot that size I fear it won't grow much more nor bloom. I hope for your sake that I'm wrong.

I brought my plumie inside for the first time last winter and put it in a northern window. They go dormant anyway, so it really didn't need the sun. I would pot them up now.

Nice looking seedling!!

Congratulations!!

Like some of us, it is a choice to wait.. Or to root now. If I lived in your area, I would root now. Buy a heat seedling mat from Dyna grow. Easy to find online or hydroponic store if they are available to you locally. Some box stores carry them, but only in the spring.

Pot them up in something you can see the roots, so you don't go poking or lifting causing any new roots to break. I like to dip in root tone/sulfur and place in a light draining mix. Something light, not heavy so water is not retained.

Water once then place on heat mat and leave it alone. It might take longer to root, but I think you will have a better chance with the cuttings being healthy now.

Place in a sunny window and keep all of the cuttings together. Here is a little pic of how I keep all of mine close together. They are all on a heat mat and stay close to keep each other warm!!

Once you see leaves about 6-8 inches water lightly or if you see roots water sparingly....

Good luck!!

Laura

Thanks for the comments/advice. It seems the consensus is to root now, which is what I have done. They are on the windowsill now and my thermometer is reading 27c/80f which is about normal for summer in our house and it rarely drops below 21c/70f. Would a heat mat still be advised?

Re: the large "seedling", I was hoping keeping it in a smaller pot may have slowed its growth, but nope, this summer it has just shot up even more.
It's getting way too tall and I can't put it outside, would it be advisable to cut it in 2 and root the cutting? I understand they only branch after flowering so I'm guessing in my climate where it's unlikely to bloom, up is the only direction this one is going?
Thanks