Why did you remove any in the first place, I am way North of you, Belfast, and I havent ever removed plants, even thalia delbata which I was told I should do and this is its 2nd or 3rd winter. The only plant I would consider removing are tropical water lilies but I cant even keep them in summer so winter isnt an issue.
I dont know when you should reintroduce them now that you have them out, btw what plants are they?, but I believe as a general rule most plants will be ok providing that the crown, growth point, doesnt freeze, ie, it is below the bottom of any ice that will form.