No - as Joe C points out, the regional and constituency ballots are now on separate pieces of paper making it impossible to pair up the ballots at the count. Indeed, the Electoral Commission's "easy guide" to the count states that the ballots are separated by contest before counting:
Depending on the space available at the count centre, ballot ...
No, because it would be impossible to enforce.
The two votes are on two separate ballots on two separate pieces of paper. It is therefore impossible to tell, given a constituency ballot, whether the person cast a valid regional ballot (or vice versa).