Pseudoephedrine is an OTC over here, you just have to show ID to purchase it and you'll often be limited to only a few days worth at a time.
I've never attempted to bring it back in with me but I have had other medicines and just marked them as medicine on my "to declare" sheet. I've never been asked to show them or anything like that.
My suggestion would be to bring it with you (not in any large amount) and just declare it. At worst they'll confiscate it and you can purchase more from a chemist.
My husband gets terrible hayfever and allergies and swears by Telfast (the green one) and Sudafed (although you have to ask specifically for the one with pseudoephedrine or they'll give you the phenylephrine one).