What I'm planning on doing for LAPL holders during transition is to give them an SEP validity only until the end of their current two years window, if you see what I mean. So, with the bloke in my example above, I'll sign his SEP up until 31 June 2026. That way when he checks his licence he won't be misled into flying in August or September this year without another instructor flight
I don't think that you have the right to restrict the new validity date in the way you propose.
The CAA are well aware of the problem, which stems from the OiD submitted to the DfT. It might well be that an exemption from the 'rolling validity' will be released for LAPL holders who have transitioned to fixed validity.