Can I go to first year Imperial CS next year if I go to a different uni this year?

I think it’s unlikely but you’d have to contact the computing department to find out.
From the Imperial website:
“Transfers are not common and are subject to availability:
The departments of Life Sciences, Mathematics and Physics may consider transfers.
Undergraduate courses in the Faculty of Engineering are not available for transfers.”

The computing department falls under the faculty of engineering so in that case transfers will not be possible.