When you say you texted did you send a private message from here as Amanda requested?
I can't see why your new card can't be blocked until your return and your existing card stays active as you initially said your existing card is valid till 12/21!!!
If @Tesco-Bank do this for you then problem solved 😁
I have received new ugly debit cards for both my accounts. So frustrating as I had until 05/22 on my existing card and now I have to change the debit number everywhere it is stored for ease of online purchasing!