If it’s an exchange then they’re legally bound to follow any public policies they’ve made or they are in breach of contract. It’s part of the implied T&Cs of the sale.
Distance selling regs entitle you to reject the goods for a refund; no ifs, no buts, no restocking fee (though they’re not obliged to cover return postage). Legally you could (and probably should) cancel the order under distance selling regulations and reorder.
If postage was £3 when you bought the original item, and there’s nothing to indicate otherwise, I think it’s reasonable to assume that they wouldn’t suddenly start doing free postage. Not sure on the legal standpoint here; I think the crux is, they can’t be deceitful about it.