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Don't think it matters, if it's not one they'll redirect you.
For the Dynamo I used the following controls (this was in the days before DIYxGPS):-
Dorking (ATM at the station), Hackney (Tesco), Great Dunmow (ATM), Needham Market (ATM), Dunwich (receipt from cafe at the beach), Needham Market (Shop), Great Dunmow (Shop or ATM), Putney (ATM).
But that, ahem, includes some unnecessary distance over the standard Dun Run. https://www.strava.com/activities/162789959
From memory the normal controls (Hackney, Great Dunmow, Needham Market, Dunwich) come out at ~170km or so as shortest distance between controls so it's not too hard to come up with something before/after to drag it over 200km.
I joined for 2023 but have yet to do any rides yet but talking of DIY's, who would I be best to get to validate a ride if I'm in Norfolk? There isn't a DIY regional rep for the East. I'm guessing either Mike K (midlands) or Martin L (home counties)??
Glad to see the Dynamo can be done as a DIY as it's on the list for this year.