Egac,
You can do as much aircraft instead of sim as you like if you want to pay for it!!!
The starting point at Bristol is MINIMUM hours / cost which is 35sim + 15a/c (if you already have CPL) or 40sim + 15a/c if you dont. This structure allows for maximum sim time credit allowed by CAA, keeps cost down and is beneficial for getting your head around procedural aspects without worrying about the actual aircraft or getting real world complications (eg ATC) before you are ready to handle them.
Remember, in addition to this you must have your 6hrs in the aircraft for MEP rating training before commencing Multi IR skills course. (Although the MEP check ride is often in practise done after the IRT).
Bearing this is mind, what you actually do is 35sim + 22a/c for an overall Multi IR course (assuming again you did CPL first).
If you want to pay to switch sim hours for aircraft (£135 vs £368 per hour) then you can!