applying for a different uni after a foundation year

To reapply with UCAS can I use my old personal statement if i'm applying to new unis?
I am not sure about this, but I would assume not since when they check it for plagarism, it may show it is copied. By making a new one, you can also add things you do more recently and it will be better than the original ps. Try contacting UCAS to confirm this though.

Does my reference have to be my tutor at the Uni, or can i use my college tutor?
You'll want to ask your tutor at uni since they are the most recent tutor and will be able to provide information from the foundation year.

If i'm not accepted to any of them can i continue my course at southampton?
Of course. If you stay at Soton, you won't have to do anything as you will be progressed automatically (assuming you get the appropriate grades). If you want to leave Soton, then all you have to do is tell your university who will remove you from the course and contact SFE. The exact process is different at each uni.

The foundation course doesn't give me any qualifications but is equivalent to 3 a-levels - where do i mention this or do i not?
Before you apply to any university on UCAS, make sure to contact the relevant department to ask whether they will accept the foundation year and what grades you need. Once you know this, you mention the FY in the qualifications tab on UCAS, no need to write about it in PS. You can however write about any modules/projects/subjects that you are interested in.

Next steps:
I would suggest you
1. Contact universities you would like to transfer to
2. Ask tutor to be reference
3. Start writing PS

Best of luck