Hi Shehnaz,
Welcome to the Forum. I hope that the MHRA will reply to you.

With regard to when the CTR "applies" in the UK (and all member states), it "applies" "from six months after the publication of the notice referred to in Article 82(3), but in any event no earlier than 28 May 2016". The Article 82(3) notice is the notice that on the "basis of an independent audit report, ... the Commission ... has verified that the EU portal and the EU database have achieved full functionality and the systems meet the functional specifications drawn up". So this means that until there is this notice from the Commission, and six months has elapsed, no Member State will apply the CTR. So it is all dependent upon the EU Portal. Until that notice is published no exact date for the CTR applying can be made.

By "summary of changes" I assume that you mean, what is different from previous UK GCP requirements? I don't know of any document like this. I know that there are some, not-for-profit professional Associations, to do with Research Quality, that have produced webcasts on what is different, from a GCP perspective. It would be useful to view them.

Laws are seldom retrospective, because of the legal nightmare that causes. I would assume that only for those clinical trials (CTs) submitted through the CTR, would the 25 year archiving requirement be applied. This would be for applicable CTs, when the CTR "applies" not from when it comes into "force" (see above).