For straight-up laptop bags, I recommend Knomo. Their quilted designs are gorgeous and practical!
Being a fan of Mulberry, a Bayswater is always a good choice and a classic style. The patent ones can take any amount of rain with no trouble and they can hold a ton of stuff, incl. a laptop. If your laptop is huge, you might want to try out the Oversized Bays. The leather is also very sturdy and won't mind a bit of rain.
As for other brands, if you're not allergic to print you could try out LV Palermo GM or Totally GM - they are made of coated canvas and have a zipper.
Longchamp does great leather bags, I'm liking this Le Foulonné tote VERY much! Longchamp leather is great quality as is the craftsmanship - I've yet to meet a LC bag that hasn't stood the test of time. Even the 30+ yrs old individuals look great (and are not cheap either, they keep their value extremely well!). Longchamp isn't a too in-your-face brand but they have that classic French charm that gets me every time...
If you'd like to take the easy way out, you could always get a Longchamp Le Pliage or Planétes Tote in the large size and use that to protect your Cerf. Le Pliage are waterproof, foldable totes that come in a large array of colors - the Cerf Tote should fit in the L size. The large one has long handles so you can easily carry it on your shoulder even when wearing a thicker coat and when it's not raining you can fold it into your Cerf.
.... Does it have to be designer?
