I have had a really similar experience! Completed first year of a geography degree, decided it wasn't for me and took a year out to take A level chemistry in one year. I'm just completing that year and have places for September for Vet med. I was also a bit worried about the age difference, but apparently up to 25% of the year can be graduate entry for vet, and most people seem to have taken a gap year too. You don't actually need to apply with predicted grades if you let them know why in a letter. You also don't actually need the practical component of the A level at most unis- everyone I asked said it was fine apart from RVC (who also wanted predicted grades I think).
I'm actually really glad to be starting the degree a little older; I know what to expect from uni, have lived away from home and studied independently, so the course will seem that much more manageable. Honestly, dropping out of my first degree was the best decision I've ever made.
In terms of student finance, you also need to drop out this year if you want to qualify for a full loan. If you continue to do a second year you no longer qualify for the entire period.
Let me know if you have any questions!!