Can you drive on a space-saver spare tyre at normal motorway speeds?

Not unless that specific spare tyre is rated for that speed; temporary spares typically have lower limits.

The NI vehicle-maintenance guidance says space-saver tyres have their own speed and distance restrictions. Read the label on the tyre and the vehicle handbook. A temporary spare is intended to get you to a tyre repair or replacement, not to resume ordinary high-speed driving indefinitely.