Mk I or Mk II? I can't profess a deep knowledge of Skol Moths but perhaps someone within this group can educate me. I will first offer up my limited, direct knowledge of these boats.
Back in early 1970s four Skol Moths were imported by sailors at the Ocean City (New Jersey) Yacht Club. These boats were referred to by their owners as Mk I Skols. During shipment, one of the boats was heavily damaged and Rondar replaced it with what we called a Mk II Skol. Both boats had similar hulls but the replacement boat had a different cockpit configuration. The three surviving Skols (of which my current blue hull is one) all had closed transoms with a cockpit scupper lead to a small square opening at the bottom center of the transom by a tube hidden behind the aft wall of the cockpit tub. The replacement boat did not have a laser style cockpit tub with the scupper but instead had a cockpit which terminated with an open transom. Interestingly, unlike the red boat which is pictured within this forum on a different post, none of the Ocean City Skols had pivoting centerboards. I assume that the pivoting centerboard is either a later modification or perhaps an early production example prior to Rondar standardizing around the dagger board style trunk. I will be very interested in feedback to either correct or uphold my assumptions.
As for organizing a UK based Classic Moth Club--I highly recommend it! My recommendation is that at first keep things simple and race any and all boats which come out to play regardless of which rig or era they represent. After a few events, hopefully the number of participants and boats will grow and some performance bracketing will prove useful. Here in the States we race three brackets: Vintage (pre-1953 designs), Generation I Classics (designs from roughly 1954 to the early 1960s) and Generation II Classics (designs after the Gen I period or new designs judged to have performance characteristics similar to narrow waterline boats such as the Duflos, Mistral and Skol. Boats which are new to us are eyeballed and provisionally assigned to a bracket with adjustments up or down as required. As noted, we have standardized around the low aspect (circle-M) rig (not to be confused with that of the British Moth which uses the same insignia). Two obvious brackets for your purposes could be divisions for low and high aspect rigs--if a critical mass of low aspect rigged boats materialize. Boats like the Shelley which started off with low rigs actually do quite well when converted to the tall rig.
As you may know, there is a French Classic Moth group: http://louis.pillon.pagesperso-orange.fr/moth/
So perhaps a friendly rivalry could be established.