I don't see anything on their email about the offer that says you have to by £50-worth. The example they give shows someone buying just one, but perhaps it's in the small print somewhere.
The vouchers expire after 6 months but old ones get extended of you buy more within that period.
The annual sub doesn't cover maps or photo collections, for which you still need vouchers.
On the subject of maps, be aware that most of the ones that have been passed to me "to help you find the grave" are just nicely-redrawn versions of the section maps usually free online, from the office, or on a noticeboard at the gate and do NOT show the positions of individual graves. There was one exception that might have come from DO as it used the same colour scheme, but even that omitted unmarked ones in a common graves area.