Can I lop back magnolia grandiflora?

You can lop it back carefully but you may not have so many blooms for a time but eventually it will flower again.Try to leave some of the promising shoots.

Any shrub or climber that flowers in the second half of the year will be flowering on wood made in the first half of the year. This includes magnolia grandiflora. The best time to cut them back is in early spring.
Anything that flowers in the first half of the year is blooming on wood grown this year and should be pruned if necessary immediately after the flowers fade. By following this regime, you should have your cake and see it bloom too!

Many stately homes grow this as a large wall shrub and so they must be pruned regularly to be kept within bounds. You won't do much damage if you prune with care. Make a note of how it flowers and on what sort of wood and follow its lead.