Your right to a cash refund because of a fault on an item ceases as soon as you have legally 'accepted' the item. This is normally at the point when you will have taken the item home and had a reasonable chance to check that everything is OK. So, in the case of a buggy, you were probably deemed to have 'accepted' the item a day or two after you bought it. You therefore no longer have a right to a refund.
You still have the right to insist that the matter is put right. (This right lasts, in the case of most goods for 6 years. Additionally, for the first 6 months, there is an automatic legal assumption that any fault which develops must be due to an 'inherent fault', i.e. poor manufacture). The retailer is permitted to choose whether to repair the faulty item or to replace it. (In practice, you'll probably be offered a replacement). The act of repairing or replacing the product should not leave you at a disadvantage so, if the buggy has been discontinued and the retailer provides a replacement from a different range, the retailer would also have to replace any 'extras' which no longer fit.
Chris