A bit of a bulletpoint post, but...
(1) You can only pay an instructor to teach you ab-initio on the Challenger if you are sole owner.
(2) If you are flying an LAA microlight you really should bite the bullet and join either BMAA or LAA, or ideally both.
(3) It's probably easier to go and get your licence on something else, then convert - which one of the other syndicate members can do for you.
(4) Let's be honest - the Isle of Lewis is a long way from most recreational flying. You almost certainly just need to go and do a residential on the mainland somewhere to get your licence. Given British winters - you might do worse than just fly out to France, Spain or Portugal and get it all done in a few weeks.
G