Birdie as standard has a plastic double chain ring and a stop clip that stops slack chain movement mounted where a front mech would be mounted.
Dont get a 105 crank with double chain ring. Shop does not know what they are talking about imo.
Two best options. Fit double standard chainring guard as per standard.

Or

Litpro make a combined outer chainring and chain ring combo for about £25 on ebay.
I like this combination as it yeids more clearance for the chain ring in folding near the rear stays. Else my silver birdy does not found well.