I'm sure you can take senekot. We can on Cambridge, but we don't use ducolax on a VLCD as it's too strong. Evidently someone collapsed on it whilst doing Cambridge, so it's not recommended any more.
Don't use fibreclear if you have constipation. Only use it when the blockage is clear...then you fibreclear each day to prevent constipation.
Using fibreclear while you have constipation will make it worse.
I know a lot of people take it without any problem at all, but other laxatives are considered safer and since they are all widely available, might aswell get one of the others