I don't think they are necessarily missing out. It depends what you do at home and it depends what you do abroad.
You can visit wildlife parks in the UK to feed koalas, the European Christmas markets also come to the UK. You can visit a huge variety of different cities and attractions in the UK, plus coastal and countryside areas, where you can learn about all manner of cultures, history, geography, art etc.
You can also do all of the above abroad and the variety will be greater, but to not do it, isn't necessarily 'missing out'. If it's not affordable or impractical for whatever reason, don't worry about it.
Some DCs might have had all manner of foreign holiday experiences, but they might never have been rockpooling for crabs on a UK beach, see the variety of geological features in places like the Yorkshire Dales, or been to some of the great small specialist museums in the UK.
If they have relatives who travel abroad, they can help bring 'abroad' to them by bringing back souveniers. I often give leftover bits of money to my nephews, who have never been abroad, they love the different shaped coins, ones with holes in, or wedges of notes that are worth about £2 in total.