If they have transport, I couldn't recommend Wales any more. There are so many campsites, well within an hour of the Wirral, we have just come back from just 1 night today.
Campsiteuk gives you pretty much all the info you could need just get searching.
Also if a campfire is a bonus then Google sites with fires allowed. We found one a few years ago at the base of Cadair Idris, a bit further into Wales but well worth it.
If no camping has been done before, a Sunday night work permitting would be a good start, as most people have gone and it's nice and quiet.
If they need any more info just tell them to give me a shout we go quite a lot. Mostly just for one night but have also been for longer.
Sorry I don't know of anywhere on the Wirral but less than an hour away is good enough to get home from if it doesn't work out.