I think you may be confused.
Before April 30 the cost for 1st class is 46p, 2nd class is 36p and a postcard is 76p.
After April 30 the prices will be 1st class 60p, 2nd 50p and postcards 87p.
If you put the wrong stamp on the postcard will probably not get delivered.
If you put the incorrect stamp on a letter in the UK, the reciever usually has to pay the difference plus a £1 fee.
