Any and every web site, blog and magazine you can get to promote a non profit aquatic plant group is key to getting new members.
If you are a member or know of a local fish society etc, go tell them about UKAPS!!!!
If you see anyone on the web from your neck of the woods, ask them to come over and join if they have any interest in aquatic plants.
I'd like to see 100 members in the group by summer(you have 40 now or so now?).
That is a good goal I would think.
That will give you a good core to promote the hobby that is well informed and able to grow things successfully and help others in the local group do the same.