Hopefully my maths is correct:
25k Alpha members would be £3M income a year (£120 each p/a)
The other £2.2M could be made from less than 62k ISA members (£36 each p/a).
I have no idea what the actual ISA take-up percentage is, but 62k people paying for an ISA from the remaining 238k non-alpha members seems feasible.