Want to change uni's after finishing foundation course. Need help

I applied to my universities via UCAS but got declined from city to do aerospace engineering with foundation year. So I am going to be doing a foundation course at university of Hertfordshire for engineering with foundation year (However this will be held at Oaklands college). After I have finished this foundation course I am hoping to be allowed to reapply to UCAS for a placement at city university to get into their aerospace degree.

I need advice if this is even allowed and what the implications of this may be.

Generally speaking, a foundation year taken at one university is not a "currency" you can use to enter a degree programme at another university. It's value is restricted to the university at which it was taken. This is because a foundation year is not a qualification in its own right, and does not earn you any UCAS points.
Was your application to City (for which they didn't make an offer) based upon predicted grades? If so, then you might find that an application next year is successful, given that you will have actual grades by that point. (It would obviously help if your actual results exceed your predicted results!) However, it's unlikely that the foundation year from Hertfordshire would make any difference to them, one way or the other.

Ask City now whether they’ll even consider that FY for direct entry.

I have, they have sent a no-reply email saying they plan to get back to enquiries within 5 working days.

OK. It seems that we'll all have to wait for five days then.