I wouldn't even consider going 2nd hand with all the advantages of the Cycle Scheme. I would recommend Fudges Cycle store in Kensal Rise as they have loads of Brommies in stock and unlike Evans are a folding bike specialist so you can test ride a few Brommies. A new one isn't that much extra on the cycle scheme and you can make it your own. Evans have the exclusive distribution on Tern for now.
As much as I love Bromptons, I really rate the new Terns and Training Wheels makes a fair point. I saw Josh Hon's influence in the higher end Dahons such as the Jetstream and Speed Pro. This has been refined and implemented even more in the Tern frames. You have chosen a very good model for the money which would be allowed on tubes and will store fine in your flat. It will also be more versatile than the Brompton if you wanted to ride on some trails off-road too.
I love Bromptons but I'm a folding bike fan first and you have chosen 2 fine contenders. The £100 accessory offer is excellent and with the tax subsidy it sounds a winner to me. If you get it we'd love to see photos and your thoughts on the bike.
Best to test ride both bikes first if you haven't already done so!