If you are on a first, then they can't be worried about academics and I expect you will get a very good reference. The only questions I expect they will have is to understand your reasons for wanting to switch and that theirs is the correct course. You just need to get the narrative right, so it is genuine, but bigs up accountancy enough to make them confident you have genuine reasons for the switch and know what you are letting yourself in for.
Its fine if you say engineering wasnt for you, but also have ready your narrative on why you chose accountancy, what appeals, what experience, what research, why it suits etc. Look at the course structure in detail, talk to some accountancy students, just have some good reasons you are in the know. They should be fine. I dont think you have anything to worry about , just make sure it really is the course for you as you only get one gift year.
Presumably your referee will know what to say and it will be supportive. Dont worry it all seems good. Doesnt seem that long ago you were doing GCSE.
Do be honest with yourself and make sure it really is the course for you because you only get one gift year.
PS internal transfers are a lot less formal because you are already on the inside.