If breathability is a concern, then it's probably still important to preserve the DWR. If you're out in it and it rains, and it wets out, then the breathability will be gone. If you're out in it and it rains, and the DWR is working, then it's going to be able to cope a bit longer.
You don't necessary have to use expense Grangers and Nikwax products constantly, Dri Pak Liquid Soap Flakes (available in Tescos) is fine for machine washing, or hand wash with soap flakes. That will preserve the DWR that's already there, and you can update it with either a Nikwax spray proofer or Granger's XT Spray Proofer as necessary.
And I definitely wouldn't bother buying the Nikwax Softshell Proof, it's just the same as the standard stuff.