The company I did my actuarial internship at was happy to offer jobs starting either 12 months later or 24 months later. It might be worth applying as though you're doing a BSc, and then if they want you to start working for them, just ask them to defer the offer by a year if you want to complete the MSci. But inform them that you yet don't know whether you want to do a BSc or MSci, so this may or may not be your penultimate year.
I was in the same position as you, where I applied for the Maths MSci at UCL just because I didn't know whether or not I wanted to do the fourth year, and it's easier to switch from MSci to BSc than vice versa. Except I ultimately ended up switching to the BSc, because I thought that if I had the internship and the job offer already, there wasn't much point in doing the fourth year. To me the fourth year was just a safety net in case I didn't get the offers I wanted, and then I'd have another go at applying in my new "penultimate year".
What you don't want to do is assume that you're doing the MSci and that this is not your penultimate year, then end up switching to BSc and finding that your penultimate year has disappeared, then being unable to apply to any internships.