Unfortunately, the podcast feed for this particular show only includes the most recent 30 podcasts. To determine that, I looked at the feed source and counted the number of
<item> tags within the
Unfortunately, there is no way to play shows older than that with a podcast player unless they'd previously been downloaded and saved to your device.
It's true that podcasters often limit this, but normally the limit would be higher — 100 is common, for example. Your best bet for a solution is to contact the show (or the BBC, if direct show feedback isn't possible) and ask them to set the limit high enough that all podcast episodes are available.