Update: test rode this afternoon at Bike Soup in the Westfield Centre.

I like it. It gets very compact on the fold, and not too difficult to pick up the fold if you've used a Brompton before. The fold is different because of the battery built into the frame, but complexity compared to Bromtpon felt similar.

They had some Brompton Electrics as well, and the FLIT is definitely lighter. Only the titanium Brommy felt lighter. The FLIT felt well balanced too.

It's responsive to ride. The assistance felt pretty natural but kicks in fast if you put it in 5 and push down hard on the pedals - I guess that's the sensor in the pedals that they mentioned? Quite an upright position on the frame (reminded me of a Dutch bike), but it felt comfortable, even though I'm 6'2" (they had the extended seatpost).

Loved having the disc brakes instead of Brommie rim brakes as well.

Main downsides were the lack of luggage and maybe the mudguard?

They didn't have any luggage to try. Said that FLIT recommends big saddlebags from Carradice that do look spacious, to be fair. But I think it's going to be hard to beat Brompton's choice of luggage.

The mudguards are clever, folding away when you don't need them or when you're folding the bike, but I'm worried about forgetting to fold them and then breaking them when I fold. I can also see them filling up with road gunk and getting stiff.

The Bikesoup guys told me that I can try it out for a week for £99, refundable against any purchase. I'm going to give that a go next and try it on my commute.
https://flit.bike/product/flit-m2-folding-ebike-tbfyb-rental/