Hi Arico, and welcome. Here is the database entry for these
Calla Lily (Zantedeschia aethiopica)
I have these growing in my garden year 'round here in Florida. They are a flowering plant (not really an Aroid, I don't think
@eclayne ?) that grows like crazy in cool weather, so they grow and bloom best for me in winter. They can go dormant either after blooming when it is hot (as they do for me here) or die back to the ground when it is cold also. Mine look similar to yours in mid-season of our very hot, humid summers here.
They grow and bloom beautifully in the Pacific Northwest (British Columbia is where I came from) which has a similar climate to the UK. Are you usually warmer in summer than there? If so, I'd recommend you grow these in morning sun, and shaded in the afternoon, so they don't get too hot. That way they will continue to grow and bloom for you further into summer.
These plants are very heavy feeders (need lots of fertilizer and rich soil) and also love water. In fact I've grown them in a pot IN my fish pond, so they can actually grow as an aquatic, or do very well as a 'margin' plant on the edge of the water as well. They don't bloom in the water but do make nice foliage.
So after all that gab, I'd say the ones you started last fall have either run out of fertilizer, or have not been getting enough water and light. They should be blooming by now, and doing better than that for you. Unlike a lot of 'house plants' I would say it is hard to over-water Callas, as long as they are getting enough light and are not too cold.
Take them all outside, as soon as the nights are above about 10deg. C and plant them in a big pot with fresh soil or in the ground somewhere with good soil, and add some pelleted time-release fertilizer. Then as the temperatures warm into summer, keep increasing the amount of water they get and they will absolutely start to bloom. The leaves do reach a certain height, and then do not get any bigger, though. That usually means the plant is getting ready to flower.
Here's one of mine, a hybrid called 'White Giant' that's been blooming since November, I think.
