Looks like a flower bud to me.

Planto

Should a single pad that's just rooted bloom?

I believe it's possible. There is some "wild" Opuntia growing here and I've never see them with more than a pad or three. Nature isn't taking good care of them, as they're under trees in the shade (hence the lack of growth after all these years). But, they bloom every Spring. I know they're still around when I see this line of yellow flowers in the Spring.

Planto

Now I don't know why it hasn't opened yet. It is possible your cooler conditions have stopped the growth. It may be thinking, well wait a minute, now it's getting cooler, time to shut down for the winter. I wouldn't be surpised if it blasts (drops) the bud.

Planto

Will want a bright location, this is a plant that grows in full sun in hot climates.

The flower bud is a carry over from when it was still part of the mother plant.

If you want, you can pull that off the parent plant and treat it as a regular pad and root it. If it's not growing anyway, and you think it might not develop further....you wouldn't be out anything.

Hmm. Well. I've started to move it away from window at night to avoid thw cold. So I would love to see an opuntia flower.

Would giving it feed with thw nkp (I have a 1.3.3 for cacti shall i use a high p?)

I'm not sure if fertilizing will help or not. If it's cooling off where you are, this might be the time of year for you when it's going to start slowing down and go dormant for the winter anyway.

Cacti enjoy being cold and dry in winter and those that do flower, most do so in the Spring because of this cold period. I overwinter mine in a cold garage when the temperatures stay consistently cold. No water and darkness the entire winter.

But, I don't know how you will be abke to grow this one where you are. You may have to overwinter it inside (I don't know if you have a consitently cold, dark area... Not freezing temperatures though!). But, winter sun is less intense than Summer and watering will cause it to grow (even just a little), and the new grow could potentially be etoliated (stretched due to lack of sun).

But,I've given you more informaiton than you asked for here. ;)

Planto

If you google 'images of opuntia ficus indica' (which is what I understand you have) there are some pics that look exactly like your flower bud, so I too think it's a flower. I had the same thing happen with a plumeria cutting. It rooted and flowered a few weeks after I got it....as the flowering was 'already in place' in the mature cutting that I rooted and grew on, as already noted above.
I would move to a warmer place but give it as much light as possible, whilst trying not to rotate it in relation to the light source as that 's what will make the bud drop if it's going too. Good luck.

Gill, also from the UK

Oh nice. Thank you. The indented part was facing thw sun? But the light is above it. Shall I tilt it?

Thank you.

Here's the flower!!

So it WAS a flower after all, and it clearly hasn't dropped off. Sorry I missed your other question above. It's even bigger than the plant. lol!

Gill UK

Woot! Thank you for sharing. :) I'm glad it finally decided to open!

Planto