Is it legal in the UK to sell a stamped envelope with a card?

Royal Mail might complain - they license who is allowed to sell postage stamps.

There are other problems though. What if someone wants to send the card overseas, or add something heavy to the card - the stamp is wrong. Are you going to give a choice of 1st or 2nd class. What about people who want to give the card by hand and don't need the stamp. What if a foreign visitor wants to buy it and take it home with them.

It doesn't actually sound like a very good idea.

It['s legal but not sensible. At 60p a stamp you will have to increase the price of your card to the retailer. Large cards weigh more so more stamps. People might just want to give a card not post it.
Not a very good business idea at all.
UK

I can see you having a few problems

Yes, provided that you have purchased the stamps legally.