The more archers there are in a club group or society the cheaper the Insurance will be per head. So a Society like
GNAS is able to obtain Insurance at around 30p to 40p per member.
GNAS usually expect you to affilate via a club but you can become an individual member (and pay more)
A small group would have to pay so much for Insurance hat their fees would need to be very high, that (I think) is why EWBS is currently asking members to get their own. When they have more members they will then look at getting Society Insurance as part of their membership deal.
It would depend on the terms of the Insurance whether you would be covered at your Club. BL-BS members are covered whenever and wherever they shoot provided the event is run to BL-BS Rules and they are using BL-BS recognised equipment. They also have affiliated clubs which provides cover if the club itself is sued for negligence.