Likewise, I've not flown any light civvie aircraft in the UK with a radio that allows guard (or any other frequency) to be monitored in parallel with the working frequency. It is not uncommon practice for aircraft with two radios to monitor guard on the second (I certainly do on long hops), but by no means a published one.
In the area you mention there are other issues as well of-course. Firstly anybody operating below the peaks in a light fixed or rotary is likely to have at-best intermittent contact with any LARS provider. Secondly you can expect a lot of non-radio traffic: mainly paragliders, but potentially also hang-gliders, PPG and microlights - none of which are required to carry a radio, and in the first three categories the pilots were never required to pass either an airlaw or radio exam either.
So, I'd guess that the effect of a TDA, promulgated by radio, in Snowdonia, is probably what you saw - some subsequent gossip but very little effect on the actual traffic.
G