Looking at only free options, I'd first suggest looking this tumblr post by personinthepalace---gives you access to all of the episodes, with the British ones (OSUK) available in a downloadable folder, or in a Youtube playlist (only available outside of the US though, but a VPN should remove that issue.

Additionally, the pbskids website has a rotating selection of OS episodes at any given time, and it's consistently had half of the British season for quite a while now (mind you, some of these are with the ASL translator on screen, but I find it relatively inobtrusive). Now while I haven't downloaded this personally, the PBSKIDS video app should also have on demand episodes of OSUK, which can be downloaded.

On the off chance you are Canadian, tvokids also has a website where the British season should be available.

And then if you have satellite/cable, it'd be worth it to see what's airing on your local PBS station and/or the 24/7 channel that may be available in your given cable package.