There is certainly a small iso 25 market. Maybe too small for Ilford / Harman to introduce a Delta 25 film. Maybe a possibility for a new Fomapan 25 film or re-introduce the OrWo Filmotec NP15 emulsion.I wonder if Foma think there is market..
You can also try the Agfa Gevaert emulsion (Rollei) Retro 80S which has an E.I. of 50 in most developers and is a very fine grained cubical type film too or pull an Acros 100 in an Ultra Fine Grain type developer. Copex, ATP1.1 or CMS20 is another solution but in fact you need a special developer for it and all micro films have problems in a high contrast light scene.