Just a couple of points as I am studying for aviation law
3) No. An aircraft that is not IFR equipped can cross an airway by day in VMC but the pilot still needs an IR and a clearance needs to be obtained from the controlling authority.
A glider is an aircraft isnt it? So a glider pilot can cross an airway in VMC without an IR or clearance?
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You should however file a VFR flight plan on that route for S&R reasons if it takes you over the Irish Sea.
Isnt the main reason that it is mandatory to file a flight plan when crossing the UK FIR boundary