I dont think lactulose is allowed, as it ends on 'ose' which indicates a sugar. Sorry!
Movicol is wonderful stuff though, it's my fave laxative when ssing. Goes straight through you without being absorbed, carries water with it then rehydrates the stool so a natural movement occurs. ie no lazy bowel like other 'stimulating' laxatives. You can get it over the counter but must ask for it.
Hope this helps!