You ask for an IFR transit and if they give it to you, they will tell you where they want you. Simple
That's why people spend half their life getting the IR - so they can fly everywhere just following instructions
You will be in CAS so you will get a Radar Control Service as soon as you cross the line into CAS (and be ready to read that one back).
They might let you cross on your original track but equally likely they will give you a heading. If they give you a heading they may ask for your current heading first.
Sometimes they ask your heading and you reply say "240" and they say "G-XXXX radar heading 240" which may sound odd (why did they bother?) but it means that your
lateral clearance is now that
heading and no longer the previous route (even if the actual heading has not changed). (the word "radar" is optional and fairly meaningless and is supposed to remind you of the Radar Control Service,
AFAIK).
In the same vein you might be at FL040 (with a QNH of 1012 applicable but you have obviously set 1013) and they might give you the transit at 4000ft on a QNH of 1012... again you may wonder why but they just happen to work in feet
That's an important bit to understand about IFR or indeed any flight in CAS. Your lateral clearance is quite separate from your vertical clearance.
But basically you just do what ATC tell you - and always remember the above para and be very clear to
- maintain your most recent
lateral clearance, till you get another one
- maintain your most recent
vertical clearance, till you get another one