back to tri again

Welcome back!  

Use what you've already got!  As long as that Brompton is safe to ride/race (bar-ends plugged, functioning brakes), ride it!  

I used to teach swimming lessons.  Your fears are the same ones the 5-year-olds have!  

Step 1 for overcoming deep water fear:  Embrace that you are only using the top 2-3 feet when you swim!  It doesn't really matter how much water is between the part you use and the bottom.  

Step 2.  Get out into deeper water, first in a pool, then hopefully in something a little murkier.  It's summer time here in the US, it's worth a few $$ to go to a public pool with a diving area and gain some confidence (check the YMCA, they may have a daily pass or give you a courtesy pass).  Go off the diving board (a bunch) so you're experienced being far under water and still getting to the surface.  Go to a swimming beach (maybe even the race venue).  

Re-think the wetsuit.  Yes, the buoyancy helps with your confidence, but the constriction may very well cause you even more anxiety.  

Get in the water before the race and get those jitters out before the gun goes off.  

I looked up your race.  The super-sprint advertises that the swim is in "shoulder-deep" water (usually in the low 70's)!!  (Not that now isn't a good time to get more comfortable with deep water.)