Or if you have space ( and I would find it difficult to believe that anyone doesn't )you could set up a temporary tub tank for him until you either have room for or can afford a glass tank.
To be fair, tub tanks are fine as permanent homes, they just don't looks so snazzy.
All you need is a large clear plastic storage tub ( wilkinsons sell big ones for a fiver, but one half the size of the £5 one will be fine ) with notches cut out of the lid for any wires to come out, a sponge filter, a small heater ( no it won't burn the plastic ) and some fake plants for him to hide in. Gravel or other substrate for the bottom is entirely up to you.
Although to be honest, most pet shops have large clear plastic tanks that are fine for bettas for under or around £10 . Or look for a second hand tank in the paper or on Ebay.
Your choices are endless and none of them involve taking the betta back to the shop.
I would also complain to the store as they should have told you that Bettas and Guppies are very well known as being very incompatible fish due to male guppies having long bright tails.