Can anyone tell me if it is common practice to change the crank oil seal when changing the clutch?

Personally, no. But may be advisable if the car is high mileage or it's leaking.

My dad's old X reg focus had a clutch change in 2014 when it was on about 80K miles (he slipped the clutch a lot) and did not have the seal changed.

I wouldn't expect the seal to be changed unless there's evidence of a leak. You'd need to remove the flywheel so unless the DMF needs to be replaced it would be unnecessary expense.

Not unless it's leaking.  It's one of things where you're more likely to introduce a leak by changing it than just leaving it.

If your in there then I would. Kick yourself later if it leaks

Seals are good these days. Years ago seals were not so good. When my dad was a mechanic 1960s it was common to have clutch slip due to oil leaking. The temporary solution was to spray carbon tettrachloride into the clutch. A number of their customers would have this done as doing the job properly was more than they could afford. I think carbontet is considered as very dangerous stuff these days. I wouldn’t replace it unless leaking, the chances of a later leak are so slim