No, a walkon in the UK is what you americans take for granted
Namely, a rec ball day where you turn up to the field with your own equipment and play.
Contrast with "punter day", which is when regular people come and play using the field's equipment (usually a bunch of suited city types doing a day of "corporate bonding *shudder*).
Since we don't have 7 million regular players in this country, there are punter days a-plenty, but walkon days are few and far between
Slowly getting better though, now with a bit of effort in most parts of the country you can probably find a walkon most weekends without driving to the other end of the country to do it.