Having flown with Ryanair and Easyjet, I know that the policy for ID clearly states what is an acceptable form of photo ID. The list clearly omits MoD 90. If the said Para had actually read his flight confirmation he would have known this.
As for his unit having his Passport, if they don't trust him to look after it, what are they doing giving him a loaded weapon to look after?
To quote Ryanair
"All passengers must present valid photo identification at check-in for all flights including domestic UK flights and routes between Ireland and the UK. Expired forms of photo id will not be accepted. The only valid forms of photo-id accepted on domestic UK flights and Ireland-UK routes are a passport, driving licence with photo, international student identity card with photo, or national identity card with photo. (National Identity cards are accepted only when issued by a Schengen country, i.e. Spain, Portugal, France, Germany, Belgium, Luxembourg, mainland Italy, the Netherlands, Greece Austria, Denmark, Finland and Sweden). "