Wyevale Garden Centres

Some time ago I received £21.50 in reward vouchers from Wyevale which I earned by spending money at the local garden centre as a member of their Garden Club. Unfortunately they have recently sold my local Wyevale to a different organisation, and my nearest Wyevale now is over 26 miles away. I therefore decided to buy something online so I don't have to make a 50 + mile round trip to spend them, as they clearly state "to use this reward voucher online, simply type in this unique code...". I therefore visited their website to find that vouchers cannot be spent online. I emailed the Company to ask them to replace these vouchers with national garden gift vouchers so I can get the value back but they refused, as below. "Unfortunately we are unable to issue your reward vouchers as a gift card or as a cash value. They would need to be redeemed at your nearest Wyevale store details of which you have confirmed you have.Please accept our apologies for any inconvenience and disappointment caused, thank you for your continued membership to The Garden Club." My vouchers are only valid until 31st August 2019 and I feel that I have been robbed, or have to make a trip which could cost me almost that value, so I get no benefit from them. This seems like theft to me and I would like to know your thoughts.