There's anold saying that "
One swallow does not a summer make " so I think you'll find that
One half a M&S lemon and sultana Danish cake
definitelty does not a high HbA1c make
You've had ONE fairly small rise on one day eating that, and it's hardly going to have a huge impact on 3 months worth of 24/7 glucose levels needed to calculate your HbA1c. Just don't do it all day every day though....
Butter and eggs are perfecty good and regular low carb ingredients and various nut and seed flours are fine too. I originally adapted my existing baking recipes to low carb versions using ground almonds or other nut flours (hazelnut, pecan, walnut, sesame - I never get good results using coconut flour though) with erythritol/stevia for sweetening, and keeping the usual eggs, butter, raising agent, spices ,floavourings, etc. I don't have any issues with resulting glucose levels.
We used to have a T1 forum member who had a "Diabetic Good Baking" web site, and we have a long term
low carb recipes thread on the forum, and various threads or posts with recipes or links to recipes.
For Victoria sponge recipes checkout
https://www.google.com/search?client=firefox-b-d&q=low+carb+Victoria+sponge+cake
or for general baking
https://www.dietdoctor.com/low-carb/recipes/baking or do a Google search for specific recipes and include
low carb or
keto in your search terms and you'll find many suitable (and some very exellent) low carb baking recpes to last you several lifetimes.