Planet X is one of them.
Something to be aware of though is because they sell bikes with a low profit margin, the admin/handling fee (approx 10%) is deducted from the value of the voucher. So a £1,000 voucher is valued at £900 against a bike you buy from them, making the top-up a little more expensive. Might not be an issue with another supplier (but this cost will be added to the purchase price).
The changing VAT rules mean you don’t save as much as before but it works out at roughly 25% off which isn’t to be sniffed at. Pity it takes so long. I’m into my 3rd week of waiting for my voucher.