You will have to risk taking some off before next spring, but I think I'd do it in the winter time, taking out leading shoots, and then give a hard prune in Spring next year, when it will bounce back without any problem.

Although Celyn Vale Nurseries are sadly no more - their site still has loads of eucalyptus info at:

http://www.blueram.net/eucalyptus/indepthguide/eucalyptushistoryintheuk.asp