Why didnt you just buy a PAYG dongle on either Vodaphone O2 or 3. About £25 to include £10 voucher so £15.
I did similar when granddaughter was in Southampton hospital but next day moved her to Marsden and couldnt use it so returned it to 3 at Churchill Sq who gave me my money back and...
Just googled it and it came up on a BBC News page the cooling off period only applies to distant selling not from a shop. That has no period whatsoever you sign you keep it unless it is faulty.