In the UK, they might like to consider Bournemouth - which has sandy beaches that have loads of people. (Compton Acres gardens is worth a visit). Click the link for a map of the area & zoom down & then drag in the yellow man for a street view to see what the place looks like. If you click & drag the map towards the west, you will see Lulworth Cove and about 1 mile west, Durdle Dor = a spectacular arch into the sea. https://jurassiccoast.org
www.traveline.info will tell you how to reach places by bus but for longer trips, people usually use the trains.
Map of Bournemouth:>https://email@example.com,-1.8612173,13956m/data=!3m1!1e3!5m1!1e1
Of course, they are more or less guaranteed sunshine in the Med whilst in Britain, it can be hit or miss. They may also be able to take day trips or weekends away to see other places in Britain that are away from their London base. So, a holiday to say Santorini beforehand, would give them a more unique experience.