Double (heavy) cream will work too, but the syllabub will be denser.
Any extra dry or dry white wine will do instead of Prosecco.
Best consumed immediately.
It will keep for 2 days, but the wine and lemon juice will separate and collect at the bottom of the glass - which traditionalists prefer. Plus the erythritol will re-crystallize, so you'll feel some granularity on the tongue.
The best way to ensure accurate measurement of ingredients is with Metric Kitchen Scales. They are inexpensive and take up very little space. Click
HERE for the ones I use. For U.S. option click
HERE.