Ring the C2W helpline and ask them. 0345 504 6444.
Basically this will give you access a network of independent bike retailers that accept the Halfords C2W letter of collection. “Voucher” is a misnomer. Be aware, that the network retailers pay a cut to Halfords out of the Letter of Collection Value, so there aren’t many (if any at all) that will sell you a discounted bike. My LBS participates in the scheme and ends up making about 5% on a C2W sale.
To save any potential disappointment or indignant “Miggins Independent Velocipede Emporium won’t let me buy a discounted bike” threads, work on the full retail price of the bike when doing your calculations. Work out what bike you want and ring the helpline. They’ll tell you where to order one from,(or tell you if it’s a brand they can’t source) or order it for you. You then print out your LOC and hand it over to the retailer.