Most airlines do the same thing
thebigben ,
When you click at the refund button on most airlines' website, it redirects you to a voucher page.
TAP Portugal is another example. Lufthansa is another example.
Airlines have to encourage people to get vouchers to ensure their cash-flow and they might often do some tricks like this as removing the online refund option and as far as I think, there is no legal requirement that airlines should have an online refund button for irregularities. BA's policy is simple and available on top of the voucher page as
KARFA shared, you need to call them.
And also, probably for personal reasons ( do not want the hotline and talk to someone, etc... ), you preferred to fill the voucher form for a refund thinking that it would work. However, as soon as you accept the voucher, it means that you didn't want to get a refund and you are OK for a voucher.
Sorry to tell you that, legal options would take you nowhere as BA probably has the evidence ( IP number, etc... ) that you agreed to take a voucher rather than a refund.