Old monobloc has 30cm of copper into what looks like an old style grey compression fit hp2o (doesn’t look like the pushfit). Then from that compression into plastic 15mm. New Ikea tap has a 40cm flex m10 or m12 (not sure which), with a compression nut and olive. Easiest route would be to cut the copper and add the flex supplied. But tidiness tells me to cut the plastic pipe below the hp2o connector and fit the compression there (adding an insert). I did consider getting a hp2o pushfit flex but they only seem to be 30cm long which would be too short for my needs. Thoughts?
Your Hep fitting is pushfit demountable. (It just was pushed on, but has the nuts so it can be demountable in the future). You should just go straight on to the plastic pipe, but ensure you have correct insert in pipe. I would consider putting full flow isolating valves at a lower level also